Keeping Food Hot?

So I’m having an outdoor wedding, but because of location the food has to be brought in and can’t really be warmed on location. This is a bit of a problem, as there’ll be about 3 hours between when the food will arrive and when the reception will be (my MIL will be cooking, not a service, which is why there’s such a large time gap). Is there anyway to solve this? I thought the serving trays with fire underneath (like you see at a buffet for a meeting or something) would be enough, but she’s saying that those won’t keep the food actually warm (what purpose does the fire serve then?).