You could set up the company in a number of ways. I would suggest forming a Partnership.
Set up the Partnership and assign each individual a percentage share. The percentage could be split up evenly or based on commitment restraints. (i.e., if one member can only commit part-time, then they could be assigned a smaller percentage.)
As projects comes in, take turns assigning the work. VA1 takes work, then VA2 takes work, then VA3 takes work. If a project entails multiple tasks, then split up the work.
All proceeds would then go into a company account. These proceeds would be distributed to each VA based on the percentage that was originally set up when the company was formed.
Ultimately, this is a decision that needs to be agreed on by all participating members. Talk it out and see if you can come to an agreement.
Best of Luck,
Julie