Well, if you were trying to win at all costs, you bench the weak players.
If you are trying to play them, but hope to win around them you put them together as a 5.0 combo. Then you only have a downside of four total games. If you don't want to stack, and you are sure anyone in the league will beat them, put them against the toughest team, as you would have a high probability of losing that one anyway, and will be ensured to win against the easier teams. (Note, if you put them together there will only be 32 opportunities for your 3.0s to play, versus 40 for your 2.5s. So if you have an equal number of 3.0s and 2.5s, the 3.0s will get less games, as you can't put two 3.0s together to offset the two 2.5s together.)
If you want them to feel like valued members of the team you pair them with 3.0s they get along with well and who will be nice to them. (Which may not be your best 3.0s...) Since you admit your team is probably 2nd out of 4, your chances of beating a team you play 3 times is pretty slim. So why not make it a successful season for everyone on the team. Put them with 3.0s at third position against the weakest teams, as see how it goes. If you have two strong 5.5 combos ahead of them you may surprise yourself and do well anyway. My first season of playing I was captain of the 5.5 combo team. I was going for the "everyone plays and this will be fun" theory of team management. We ended up at Sectionals anyway.