You have a couple different options.
I think the cheapest would be to switch to a compact crankset which would change the front chainrings from 52-39 to 50-34.
I'm pretty sure you could do this while still using the same shifter.
Changing the front shifter is required if you want to go from a double to a triple, as is changing the front derailleur.
If you switch to a compact I'm pretty sure you can keep the same derailleur and shifter.
Another thing you'll want to look out for is the rear derailleur.
You currently have a short cage derailleur, and you may need a medium or long cage derailleur when you go with a compact crankset.
I don't think a triple should really be necessary on a 10 speed system.
I have triple, but I have an 8 speed cassette.
I find in that case the triple is nice because you can still get lots of gear range while still keeping the gears close together, However, on a 10 or 11 speed cassette, this isn't really necessary.
I think that switch to a compact crankset is a much better option.
Also, according to This post , the maximum cog size of a short cage Shimano short cage is 27 and the total drivetrain capacity is 29.
To use a larger cassette, you would have to switch out the rear derailleur.
If you used a compact crankset, you should be probably also replace your rear derailler for a medium cage, as your total capacity would be (50-34) + (27-12) = 31.
However, if you changed the cassette to a 12-25, you could get away with not replacing the rear derailleur and still getting a little bit lower gearing, although possibly not enough for your needs.
bicycles.stackexchange.com/users/163/Kibbee
You'll need to swap out your left (front) shifter and front derailleur to utilize all three chainrings of a triple.
Between the cost of that and the cranks, I think a better option would be to swap out your rear cassette for something with more range.
Since you've already got a 12-27 on there, a 12-32 would give you
I believe the range you need.
My hunch is that you'd be able to fudge it with your current short cage rear derailleur, but a medium cage mechanism will probably be less hassle in the long run.
Since cassettes are consumables and need to be replaced periodically, this cost of the just rear mechanism is likely significantly less than the shifter, front mechanism, and the crankset.
bicycles.stackexchange.com/users/5918/Paul H
When you say two lower gears, you presumably have 25 and 27 as the largest gears in your cassette.
Does that mean 39/31 would be low enough?
If so, the compact with a 34 will get you there, but 34/25 will be too high.
Shimano makes a 12/30 10speed cassette in the CS4600 line, which I have coupled to a 50/34 compact.
You will be in for a new rear derailleur that way.
They also make a 11/32 11speed, but that needs new shifter as well.
This might avoid the crankset change, however.
bicycles.stackexchange.com/users/10998/Ross Millikan
I wouldn't get a triple crankset, there's a reason why those are dying off of the market.
Consider instead changing your cassette and maybe your rings, if you're climbing a lot of steep hills and want a lower gear ratio, then what I would suggest trying first is swapping out your cassette with a mountain bike cassette, this may require you to also change your rear derailleur to a long arm derailleur, and you'll need a new chain, but you should change that whenever you change your cassette anyways.
If you need a little bit more, then try swapping your small ring with something smaller.
You'll save money this way too, because you won't have to change out your crank or your shifter.
bicycles.stackexchange.com/users/14392/ShemSeger
When I made this change 15 years ago, as well as the new triple chain rings I needed to change the bottom bracket - cotterless square taper type - (which needed to be longer to correct the chain line), front derallieur (needs to be able to cope with extra travel and larger difference in chainring sizes) and rear derallieur (needs to be able to cope with larger total difference in teeth).
You will also need to change your shifter to use the new range on the front.
bicycles.stackexchange.com/users/4447/DanS
I have a  2007 Specialized Tarmac Pro Double  and I need some better gearing. I am doing more hill climbs and I feel that I need at least two lower gears than I already have. What would be the most efficient way to upgrade my bike. Would it be easy to get a triple chainring? Will I have to get a new derailleur? What else would I need to change?
What is the easiest to upgrade my chainrings from a double to a triple?
bicycles.stackexchange.com
<road-bike><chainring>
bicycles.stackexchange.com/questions/31030
bicycles.stackexchange.com/users/14679/Zlatty
There is some degree of disagreement about whether a new derailleur would be needed when moving from double to triple chain ring. Some think that by using a compact crankset, the original derailleur could be kept while others believe it would, nevertheless, be necessary to change the short cage derailleur for a medium or long cage. Others consider that even more part would need to be changed, for example the shifter. A triple chainset could require changing the bottom bracket as well as the front derailleur and shifter. A compact crankset would be cheaper and allow you to keep most of your current components. You could also change the cassette to something more suitable to your needs.
A triple chainset could require changing the bottom bracket as well as the front derailleur and shifter. A compact crankset would be cheaper and allow you to keep most of your current components. You could also change the cassette to something more suitable to your needs.