It depends on the route: if there are a large number of status passengers, those on full fares, those on corporate deals and/or a lot of people prepared to pay for seating then there is a chance you may be split up. If none of that applies, and perhaps it's not a full flight anyway, then Theoretical Seating should keep the three of you together, worst case scenario being that one of you would be over the aisle, but otherwise contiguous, to the other two passengers. If your group has a child or a disabled person in it then you can ring up to sort out seating 3 days before travel anyway. Being kept together isn't guaranteed but it generally happens these days on short haul. It's very rare to see shuffling around for families since TS was introduced. (MCO and TPA would be exceptions here).

For LGW, as someone who travel hand baggage only and knows the airport well, I plan to arrive 1 hour before departure, which gives me a bit of leeway for train delays (etc). So if you have checked luggage then arriving at check-in 90 minutes is more than sufficient, though perhaps a bit more if you don't want to use the self service machines and it is at the start of the school holidays.