if you cast a spell on someone 3 years ago, it only effects that person, it doesn't automatically go to the person you like right now. if you want to use the same spell but on this new person, yes, there's no problem. unless you use the exact same materials. like if you used a pink candle in the spell 3 years ago and you still have the same pink candle, i wouldn't use it. i'd get a brand new one.
if you want to cast love spells on 2 or 3 different people at the same time, you can do that as well, but be sure you charge and focus enough for each one. anything magical drains your energy, so i'd do them spaced out.
if you mean cast one spell but say 'i love you, love me too john, dick and harry' i don't think so. never done one, but spells need to be as personalized as possible so you can direct the energy towards the right person and get the right outcome. so in this instance, no. cast 3 different spells. they could be the same 'make him love me spell' but get 3 sets of ingredients, cast it on 3 separate nights, and above all charge and focus. if you feel weak, or tired, leave it.
apart from that, cast it under the right moon phase, and keep in mind love spells cast over another imposes your will onto them, so they might love you only because of the spell. so as always, be careful what you wish for.