If you’re doing a Shalosh Seudos, your guests may still be fleishig from lunch, so I would recommend making it pareve (that’s what we did for my brother’s Bar Mitzvah Shalosh Seudos last summer.) Serve simple food like bagels and egg salad, but make sure to have enough quantities for everyone. Also, put out lots of cakes and cookies. If you want to be more adventurous, try a fruit soup. An easy salad to make is just lettuce with red onion, mandarin oranges, and candied pecans. Use a simple dressing so as not to overwhem the flavors of the ingredients.