Sounds like the toilets are leaking through the flapper valve.
Open the lid on the tank. When you flush, the flapper lifts up, letting the tank full of water down into the bowl.
If you put some food coloring in the tank water a change in color in the water in the bowl will indicate that it is leaking through the valve.
It is a relatively easy process to replace the "guts" of the toilet. If you have to take one thing apart, you might as well do the whole thing. My experience has been if you replace one leaky washer someplace, the act of working on the toilet, causes at least one other component to decide to go bad on you.
A new flapper valve is only $2-3 maybe $5, while a whole new set of guts will run about $20. The frustration with taking it back apart to fix something a second time is well worth that in my opinion.
There are some good step by step instructions (even videos) of how to go about this type of repair online. Start w/ big box stores, mfgrs, This old House, HGTV, etc.
Good luck. It sounds like you have a couple significantly leaky toilets. Repairing those leaks will save you quite a lot of water.