Skizzle,
We had a league like this when I lived in TX. It has faced similar problems, as well some dumb rules that were voted in to try and stop sandbagging. We used to always vote on a cap every year, so it was variable. We usually went off the league average for the year just finished. You could do the same, find the league average for entire season for the 4 man teams, divide by 4 to get the individual league average, then multiply that by 3 to get the new 3 team league average. It's a starting point and then you can discuss/debate from there if you need to go up or down. This works best because it takes into account any center advantages/disadvantages. Some centers the scores are lower due to age of lanes, maintenence of lanes, oil patterns, etc., so saying this cap would be right for the league is hard to do.