Hole Pattern as you can do fill so will be equal or Locked Dimensions and Formula for the holes
I draw 3 lines between the points and make the lines "equal". (= sign)
This is in ordered.
In either mode, if you don't want to use a pattern, you can use @ChrisMorgan's 2nd solution of dimensions with formulas. I would just make two of the dimensions equal to one of them or make each dimension equal to 1/3 the overall.
I use joris' method alot or else I make a pattern in a sketch first and then pin the hole to that sketch. Then pattern the hole from the sketch.