Hi, where do you plan to put your trellis? What sort of squash are you growing? If your trellis is down the middle your plan will work, but you can technically fit a lot more in the top row--according to Mel, a trellised tomato can be contained in only 1 square foot. It does require some pruning, and of course you will need to tie the tomatoes and the squashes to the trellis, so make sure it's a very sturdy one.
If this were my garden I would put the trellis along the 7' side (assuming it runs north-south). Then put one of the tomatoes into a separate container and put the squashes in the empty squares between the tomatoes. This way you would still have 2/3 of your garden space left to plant! You could put in pepper plants, swiss chard, broccoli, spinach, bush beans, etc. (larger plants) in the middle row, and then the front row could be lettuces, radishes, carrots, fairly low-growing herbs, etc.
You will be shocked and delighted by how much stuff you really can fit into just a 3x7 space!!