To be fair, they did get some pretty decent damage in on their surprise round. They just didn't get many more chances.
5 level 3+ characters vs 3 CR 1 bugbears and a handful of CR 1/4 goblins.
That is a trivial encounter. Of course you wrecked them.
Give 'em hobgoblins; if I recall correctly, those guys have like 18 ac and teamwork abilities. They're also really low CR; my brother's party faced a group of them and could barely hit them. Cheap for sure, but a guaranteed difficult encounter.
If I were a player in this game I actually would be worried. The DM doesn't seem to have a clue how to make a balanced encounter and after this is likely to just look up random Monster Manual entries of a CR six levels higher than the party or throw so many weak ones at the party that the action economy makes it impossible to survive.
A sneaky DM trick that can do a little better when reeling like that is "HERE COMES A NEW CHALLENGER" where the pushover battle has a slightly more difficult wave of reinforcements afterward, and maybe one more after that with some extra treasure incentive for it to be seen as a challenge rather than a punishment.