2 or 3 per year does seem more likely, both based on my personal impression and on the numbers that quick Wikipedia search yielded.

And yeah, people's general perception is warped by the simple fact of the US having so many more inhabitants - many European countries have just one notable mass shooting in recent history, or none, but then they may only have 5% of the American population or less. There's also a perception that the shootings, particularly those in schools, are "imported" from the US, fall-out from Columbine and all, which I suppose further distorts the impression about the numbers. Of course, tons of things are "imported" from the US to Europe that way, and it's only the bad ones that are generally acknowledged as such, as one might expect. Your guess sounds plausible enough to me.

As Nate pointed out, the non-spree murders would be a better point to focus on - the US having a homicide rate of multiple times that of most European countries, much of it with guns. But whether tighter gun control would appreciably bring down that homicide rate, is obviously a different question. You could avoid certain categories of homicides, I do think, like that shooting in Florida earlier this year - take the gun out of the equation and nobody dies there. Some other categories, not so much, and as pro-gun folks keep pointing out, a gun in the hands of the right person can occasionally save lives as well.