I can guess that ３つくらい its meaning something like "3 of something" But what's it "something" in this sentence? Floors? Or house? And I don't know anything about ベシュワー, but I can guess that it's sound when using a spray? Thanks for your help!
Peshuu~ is like 'Psshhhhhsh' sound coming out from a can. And 3つ might be みっつ japanese counting of 3. As in ひとつ、ふたつ、みっつ、etc.
くらい Is approximately, so the meaning of 3つくらい is 'approximately three (things) '