AIBU to think this couldn’t be possible?

[Bloodless]

So I’m looking at flights for next July on Skyscanner, there is BA one that involves a change of airport in London.

Belfast - Heathrow - Gatwick to USA

The stopover time is exactly 3 hours, is it possible to do that in 3 hours? Assuming bags would have to be collected at Heathrow and checked in at Gatwick.

Has anybody done this before?

Wouldn't risk it. Have you looked at flights from Glasgow? Cheap flights with budget airlines from both Belfast airports and I daresay Derry too, direct flights to Orlando. Or you could even get the ferry over.

The trains would be a pain. Its the heathrow express (quick and reliable, but expensive, to paddington, that takes 15mins and is every 15mins. Then the tube from paddington to Victoria, and then Gatwick express.
That's 2 changes, and anything could go wrong at any point. It will also be expensive (the heathrow express alone is).

Honestly I wouldn't do it and not with that time limit. Heathrow can take an hour to get out of (if its busy and you get stuck circling, and then taxying and then have to collect bags). That gives you 2 hours before your flight leaves and you've not left the airport yet. You then have 1 hour to get to Gatwich - possible but not realistic, because don't forget that will need to be at the airport at least an hour before the flight leaves (for security, passport checks and bag drop).
If you do it, I'd do as other's suggest, cheap airport hotel at Gatwick and spend a night there, or get a much bigger gap between flights.