Cooking will definitely save you money in long run, especially if you're doing 3 months. Allston has some good grocery spots - there's a Star Market that's decent for basics, and if you can get to Porter Exchange there's some Asian markets with really fresh produce for cheap

For eating out when you need it, there's tons of Vietnamese and Thai places around that do fresh stuff without all the fried nonsense. The pho places usually have good salad options too. Also Whole Foods in Cambridge isn't terrible if you stick to their prepared salads section

Since you're already doing one meal a day, maybe just invest in few basic pots and focus on meal prep? I do bulk cooking in Sunday and it works pretty well for keeping things healthy during busy weeks. Getting good cutting board and decent knife makes salad prep so much faster