So, er, I don't know how to wizard. I could use up a whole spell and cast sleep at the two together? Or just shoot them with my crossbow. It seems a waste to use a spell when I have only 3.
First off, hobgoblins are human+ sized, so barring an unusually tiny hobgoblin, even if your diplomacy attempt succeeds, the "shoot the hobgoblin in the face" plan is still probably pretty good.
Second, spell usage can be a bit finicky, and depends on things like how often you'll be in combat encounters between rest periods, how reliant you are out-of-combat on utility spells, party composition/capabilities, how likely you think the enemies are to make their saves (if any) and more. You'll get the hang of it with practice, but particularly when starting a new campaign with a new DM it can rquire a fair amount of trial-and-error.
One thing to take into account is you're a decent Int character and an inexperienced player, so try asking the GM what your character would know about the situation/enemy. This is the kind of thing Knowledge skills come in handy for, by the way.
Personally I probably wouldn't use a Sleep spell here unless either they grouped close enough to hit them all, I was sure there wasn't going to be much at all in the way of difficult fights before rest, or I had some particular in-character reason (such as making a strong first impression), but I am somewhat conservative when it comes to resource allocation and do have a tendancy to be more stingy than I should be from an efficiency or even safety standpoint, so a fairly large pinch of salt there.