The 3 heroines is add-on characters that was later patched into the original game. Therefore they don't have an ending, and you can't fill the love meter the normal way.
You've to use the baby making option, every time you did that the love meter will increase by 2-3%. You don't need to fill it to 100%, if you've succeSSfully impregnated them in the mini-game, the love meter will be maxed automatically. However, since they don't have an ending of their own, all you get out of it are just the children. (Roboko daughter is very powerful, though.)