I think I know what you mean, I've faced the same issues from time to time.
As I understand you're not a professional singer.
Yes, a voice should become stronger over time as you train more.
The muscles that control your voice will indeed get a bit stronger but to be honest
I don't think singing should be compared to weightlifting for example.
It's more about mental training and proper coordination.
There's still a big warning sign.
When somebody sings the voice should not deteriorate but instead it should improve and get warmed up over time.
That alone tells me that it's likely that there's something wrong with your technique...
it could be anything.
It's common when someone overuses their voice that it feels "tired" for a sort amount of time afterwards.
After all, if you're still not a pro singer you can't really know if you're singing right unless you sound good and face no problems.
Until then you're just guessing.
music.stackexchange.com/users/10085/user40079
Singing is of course something requiring training and/or endurance.
If you ask "how do I get to run a marathon", the answer will involve lots of running.
That being said, if you are hoarse/strained after two or three songs, that's comparable about being shot after going up two stories of stairs.
If that's your starting point of doing a marathon, either you are doing something quite wrong and/or
you are woefully out of shape.
"Just do more and it will come naturally", just like with the running analogy, would appear to be reckless advice then without looking closer.
Try getting some actual supervision for a while.
It does not necessarily need to be one-on-one voice training: some choral work or basic singing courses might be enough to have somebody have an ear on you and
have you avoid doing staggeringly counterproductive things.
music.stackexchange.com/users/13191/user13191
I have the same issue.
A friend of mine gave me a tip which helped me.
Try to sing from deeper down your throat.
It is hard to explain, but basically, by doing so, you will stress your upper part of the throat less.
music.stackexchange.com/users/9975/Valentin Grégoire
This might help, in my experience I've sang my heart out for a very long time and my vocal endurance slowly built up.
I've always heard you need to workout to be a good singer
but I had lifted weights for a while too
but my progress to build vocal endurance was still slow.
I've recently (7 months ago) injured my wrists and I'm still recovering.
The doctor told me to I need to do aerobic exercise to help my healing.
This was the key for my singing.
After heavy endurance running and biking I started to see a large improvement in my vocal endurance.
I've heard from my vocal teacher in college that when the body gets tired, the voice is the first to be affected.
Having high arobic endurance means it takes much longer to get tired.
Also check your diet.
There are certain foods that seem to produce mucus on the vocal cords.
If you are eating these foods before you sing it would cause mucus build up.
Singing with extra mucus is a sure way to wear your vocal chords out quicker.
I hope this helps everyone out
music.stackexchange.com/users/24561/Nairda
Of course it takes time for your voice to get used to a lot of singing, but you can't compare it to running a marathon.
You just don't get better by doing it.
You get better by knowing what you do, discover mistakes in your technique and directly working on those problems.
If your voice is strained after a few songs, you propably have a problem with your breathing, which is propably the most important aspect in singing.
You may feel that you don't push or force the notes, but the fact that your voice starts to crack that fast tells me otherwise.
We could exchange a lot of tipps and how to do it right in theory, but in my opinion you won't get the results you want from a board on the internet.
If you really want to sing and don't want to ruin your voice, go get a vocal coach.
I know, they cost a lot of money, but it's worth it.
The right teacher can help you improve your voice and your overall singing experience a lot, so it's worth the money, even if you are no professional.
music.stackexchange.com/users/24260/Matthias Nicklisch
You feel tired because you aren't using your voice properly.
Do you use correct breath support?
Are your muscles trained and strong?
Are you singing according to your physical region (bass, baritone, tenor?).
To answer all these questions, and to reach your objectives, I recommend you to search for a good teacher.
I found a great singing teacher and it changed my life forever.
We can only get what we want if we dedicate and practice a lot (with correct supervision, of course).
Hope this helps.
No shortcut is good in the case of voices.
You can damage your own if you don't practice correctly.
Believe me.
music.stackexchange.com/users/24574/Raffael Bechara Rameh
I have good technique. I'm self taught but I know all about singing and its techniques. I can use head voice and I'm comfortable with it. The problem I have is that I can't go through two or three songs without my voice starting to crack or just feeling tired. I don't force notes just so you know I try to sing as comfortably as I can.  How can I build up my voice's strength so I can last through a whole concert?
how to build vocal stamina
music.stackexchange.com
<technique><voice>
music.stackexchange.com/questions/23161
music.stackexchange.com/users/12410/user3789596
S6: If you really want to sing and don't want to ruin your voice, go get a vocal coach. Also you can try to sing from deeper down your throat. There are a number of things that you might want to consider to build up your voice's strength. However, the most effective will probably be to seek a vocal coach. Asides from this, you could try singing deeper in your throat or increasing your aerobic endurance.
You might want to consider getting a vocal teacher to get to the root of the problem. You might want to work on your aerobic endurance. You could try singing from deeper in your throat.