THe thought of 3.5 hours squash terrifies me!!
If you have a round in the morning, another one in the afternoon, and further rounds in the evening, i would have thought eating what you normally eat, except for smaller portions, would be best.
On the few occasions (awwww.....) when we've got through several rounds in an evening all the way to the final, i've found that
1) sports drinks are essential
2)chocolate snacks (such as 'jaffa cakes') are great
3)bananas are good too
4)something to keep me calm and relaxed is a great help.

I must admit, i've not done controlled experiments