If the game starts off at level 1, I can't see how they'd fit +5 weapons in unless the game is absolutely epic?
Comparing to BG 1 and 2. I think the best you get in BG1 is a +3 from memory? You only get to +5s when you get to Carsomyr etc in BG2.
The best you can get in BG1 is the
+5 scimitar from the gay black elf. But, yeah, aside from that single easter egg, the highest enchantment you can find is +3 (a quarterstaff and a two-handed sword, both in the expansion). There are, however, a lot of +2 weapons and some of them are extremely easy to find at the very beginning of your adventure.
BG3 will end at level 10, so "high level" items shouldn't appear at all. Also, even if the game goes absolutely epic, in D&D 5 even the most powerful artifacts don't go over +3.