Bring all three, but for portraits on an APS-C Canon, the 50mm f/1.8 is usually the best choice. It gives a field of view close to an 80mm equivalent, which is a classic portrait range, and it’s sharper and much faster than your kit and telephoto zooms. That makes it especially useful indoors or in dim light, where the wider aperture helps keep shutter speeds high enough to avoid blur.

Use the 18-55mm when you need flexibility: small groups, tighter indoor spaces, or environmental portraits that show more of the room. If you have enough light or can use flash, it can work well for party coverage.

Use the 55-250mm for candid shots from farther away, but it’s less practical indoors because it’s slower and needs more working distance.

If you only have the built-in flash, try to rely on available light when possible and use flash sparingly as fill. For a birthday party, the most practical approach is: 50mm for individual portraits, 18-55mm for groups and wider scenes, 55-250mm for distant candid moments.