bearded dragon without heat for 3 hours in garage in uk be ok?

Fill up warm water bottles with boiling water and wrap it in towels, your dragon will get all his heat from there. If you dont have warm water bottles then you can use ordinary coke bottles, but remember to wrap them first. As the bottles cool down you can remove the towels and he can be directly on it.

Are you 100% sure about the 3 hours? Are you going to put fans on to get all the toxic air out?

wrap a hot water bottle up in towels or dish cloths (i do this when i take my lizards out in public or have to move them) put them on one side of the viv so they can cool down because the water bottles can often be quite hot, make sure the towel is thick so his claws cant burst the bottle because that could burn him. Make sure you keep the viv out for a few hours afterwards dont take any risks with his health :)

That depends on how cold the garage is.

If you feel it will be too cold, or worry about him possibly getting cold, you could fill up some hot water bottles and put them in his tank. That way, he'd have a heat source just in case he needs it.

During the winter my beardies endure 55 degree temps for all nighter. 3 Hours or so they'll be fine

well in the wild they endure quite cold nights due to them living in a desert so i guess he'll be fine

Only if its for a lil while