I would probably put the weakest two guys together at #3 but might occasionally put them with a stronger player but still have them play #3 either way. They need to win at #3 first before you move them to #2. Putting them with a stronger player should help them learn through experience but could frustrate the stronger player too if opponents hit mostly at the weaker player so rotate them with different partners if you choose not to play them together.
You need to get them in 4 matches so play them together twice and then put them with stronger partners at #3 the other few times. Let them play in 5-6 matches minimum as having fun and making friends at the 2.5-3.0 should be the rule and not just trying to win all of your meets.