For calculations of developer capacity (any developer) 1 roll of 120 is equivalent to 1 roll of 135-36 is equivalent to 1 8x10 sheet is equivalent to 4 sheets of 4x5.
For X-Tol in particular:
1) 3 litres is a really large volume for the working solution, unless you are doing something like developing 8x10 sheets in big tanks. One would normally use a working solution container that is slightly larger than the developing tanks one is using. It should still work, but you might need to put a lot of film through it to counteract potential problems with oxidation;
2) Kodak recommends that for each roll of film you develop, you add 70 ml of replenisher from the replenishment stock to the working solution bottle while the film is being developed. After completing the development you should pour the recently used developer back into the working solution bottle. It will over-flow if you pour it all - discard the excess.
3) as XTol is its own replenisher, it is difficult to over-replenish. For that reason, some people use more than 70 ml per roll - 80ml or up to 100 ml. Of course, if you use more you decrease the total maximum number of rolls that a 5 litre package will develop. You might decide to increase the amount of replenisher if, over time, you notice that the developer activity is slowly decreasing;
4) at 70 ml per roll, you should be able to develop 70 rolls of film from 5 litres of replenisher. Kodak suggests a working life of six months for mixed up replenisher. So if you develop a roll every two or three days, you should be able to use up the replenisher in that six months. Then you mix up a new 5 litres;
5) with respect to the working solution, if you are using and replenishing it regularly, it can theoretically last forever. The key is regular replenishment - remembering of course that replenishment involves both discard of a portion of the old and addition of the fresh and new. My practice with replenishment is for any week that I don't develop film I discard one film's worth and add one film's worth, as if I had developed a film;
6) if you replenish, you don't dilute. If you dilute, you don't mix the diluted developer into a replenishment regime. In your case, with all that excess working solution, you could consider reducing its volume by replenishing the whole and than separating of an amount - say 1.5 litres (leaving 1.5 litres as working solution) and then using that separated off 1.5 litres for one shot development with 1+1 developer. The separated off developer will be slightly different than fresh developer, due to the single roll you developed in it already, but the difference won't be great;
7) D-76 isn't set up to use replenishment in the same way. It requires a separate chemical called D-76 replenisher, which is no longer being made. Ilford makes a close equivalent - ID-11 - which is packaged in two parts. Because of that difference in packaging it is possible to customize the mixing and prepare a replenisher instead. Ilford has instructions for that on their website;
8) some people are (overly) concerned about not being able to use up 5 litres of X-Tol in six months. Considering how inexpensive it is, and considering how relatively benign ascorbic acid developers are, I'm not sure why discarding a small amount of developer once in a while is so bad. Some people have had success storing X-Tol safely for more than six months, but you need to be willing to do clip tests, because X-Tol doesn't change appearence when it dies.
One of the really big advantages of using replenished X-Tol is that it permits you to always develop at room temperature. Just measure the developer temperature, and adjust developing time accordingly.