How close together do beach plum have to be? And how long until you get your first crop?

I've been buying them for a few years and squeezing them into open spots. They are kind of in a triangle. There are a pair of big ones 52 feet apart, 87 feet away is another one, and a small bare root one 160 feet away from them. They were two in one gallon pots planted in 2012 and 2014, plus a bare root one planted several years before. No fruit so far. Are they too young or too far apart?