The way it is now, PL has an advantage on one front (the one with the neutral base) while RF has the advantage on the other two - the corner and the airport fronts. This gives FTA because RF will always have a slight unit count advantage, not to mention a positioning advantage on two of the three fronts. That PL is ahead on the most important front doesn't make up for that advantage.
If you add an extra PL infantry on the corner base, all that happens is that now PL will have the advantage that RF currently has. It doesn't even it out, it just shifts the advantage.
But if all 4 bases are preowned, then PL goes first from 2, RF goes first from 2, and everything is more balanced. It still won't be perfect because the fronts are uneven, but it should be more or less negligible at that point. Also, having 4 bases preowned isn't that big of a deal. It's a big map with lots of funds, so there shouldn't be too much infantry spamming.
If you insist on 3 preowned bases, then the best way to do it is with a preowned city for PL within 3 spaces of RF's corner base. It's less perfect than having 4 preowned bases, but is better than the current setup.