That would mean something if Knapp Street had a decent catchment area in the first place. By the time you hit Avenue X (heading south), what little potential it had to the east bottoms out, and the west does the same at Avenue Y. North of Avenue W, you may as well not even bother, as the B3 is at Avenue U (heading south, this is the last street to connect Flatbush and Nostrand Avenues).
Sheepshead Bay covers much more than just that station. Depending on where you are, you can literally just take one bus to Flatbush Avenue and transfer to the Q35.
If you used Trip Planner for that, the trip time would be an estimated 50-60 minutes, not 90. It would come down to the transfers and fares.
The most notable issue, though, is the fact that this is a route that would only perform modestly during the summer, whereas it would tank during the rest of the year, especially if it uses Knapp Street.