Window Installment Help. Installer Split. Have to finish the job myself quick.?

Here's the story. A guy I used to work with convinced me he'd install 3 windows in my house for a good price. He came over and told me to order three windows size 27.5 X 53. I ordered the windows from Lowes and spent $550 bucks. I didn't really have the money to spent on these but want a warmer house this winter. Anyways the windows came in and he came over to install them. He started the first and second window, they are beside each other with a column between, and then started saying I ordered the wrong size and that they are too narrow. The windows are in and there is about a centimeter gap between the edge of the window and the outer wood trim, that the window is supposed to be up against. My problem now is that the guy has split and i'm stuck with 3 windows, 2 installed and not even caulked and not much money left. I have just researched installing windows on the net and from what i've seen I could handle the install myself. I'm handy just never attempted it. My ruff opening is 28.5 wide and the windows are 27.5 wide. The height is fine. This leaves me 1 inch. So basically My windows are a half an inch to small on each side. What can I do to get these things installed correctly? Is there some sort of filler strips I can place along the sides of the window opening to fill it in? Also any suggestions on the inner and outer trim gaps? Any help is appreciated, Winter is almost here and I need to fix these correctly. Thanks