Personally, I find buffets are the easiest provided meals for me, as I can choose exactly what and how much to eat, and bolus for it. The only issue is guessing carbohydrate quantities - I practice occasionally by weighing food at home & memorising carbohydrate, so I can recognise portions when eating out. If in any doubt, I prefer to inject less and if necessary, have a correction dose a few hours later. Low carbohydrate nibbles are good for eating without injecting. So, my advice is ask everyone for any specific food requirements / likes / dislikes - type 1 diabetes doesn't really bring any specific requirements, but people with type 1 diabetes can have preferances. If any of the three have type 2 diabetes, then offering lower carbohydrate foods and drinks is more important. You might want to offer diet soft drinks, as fruit juices and normal soft drinks are high sugar; alcohol is OK, as long as you're sensible and not driving.
For example, today I was working, 11am - 1pm organising an orienteering session, so lots of running and standing oround outdoors, so had a muesli bar before start, then a sandwich after finish; afternoon was an orienteering party for 7 year old children, 2pm to 3.30pm, followed by tea - was offered and accepted (was starving) hot dog sausage in roll, baked cheescake, sticks of carrots, peppers & cucumber. Plus, normal 3.5 miles cycle in each direction and 1 mile foot patrol before 11am. About time to check blood sugar before tea, being cooked by my partner as I type, lucky me 🙂