The chances are obviously of better quality but in 15 minutes minimum last season, 3 goalies didn't give up a goal and 12 goalies (including Sammy) had a .900 or better SV%. So it's not like goalies can't stop anything in 3v3.
Median SV% is in the .850 range. Goalies are always at an advantage in every tracked type of shooting situation.
Now, with all of that said. The Leafs are fucking brutal in 3v3. They allowed 52 shots per 60 minutes last season, and only took 26 for. 2nd worst shot share in the league in 3v3. That we're only mildly below average in goal share is fucking amazing and a testament to how talented our OT shooters are when they get a chance.