City of Lincoln Council said charging for facilities at Lucy Tower Street, Castle Square and Tentercroft Street could bring in about Â£30,000 per year.
It said the changes were part of a range measures to meet savings targets of Â£4m per annum by 2018/19.
The council said the charges were aimed at getting tourists to contribute.
John Latham, from the authority, said: "Lots of places you go to you have to pay for the toilets, and of course, the expression 'spend a penny' came from that tradition of having to pay for public toilets.
"We think it's reasonable, given the number of visitors, that those visitors help to meet the cost of these facilities by making what will be a very modest contribution."
A report by the council said: "Increasingly charging has become recognised as a way by which visitors to a district, who impact on local infrastructure but pay nothing directly towards its upkeep, can financially contribute to the costs."
The cost of installing barriers at the three sites is estimated to be about Â£30,000.
City of Lincoln Council, which has a total of eight public conveniences, said all three of the pay toilets are highly rated by the British Toilets Association.
A council has approved proposals to charge 20p to use three of its public toilets as part of plans to combat government funding cuts.