This is an excerpt from one of the pocket St. Petersburg guides:

A “troitsa” is a threesome and not to be confused with “troikas”, those horsedrawn sled carriages that used to romp around parks in the snow and in Tolstoy novels. Troitsa comes from the same root, the Russian word “tri”, meaning three. The original troitsa is of cousre, the trinity of the Father, the Son and the Holy Spirit. Hence the following phrase “Bog lubit troitsu” – God loves a three. For instance, if your grandma is giving out lollies, she’ll dole out three, not two, for each kid because… God loves threes. Or if you’re doing some vodka shots, why stop at only two?

That’s what you thought they meant by threesome, right?