I dont know if they would qualify as 'similar'. After all, they are effecting different stats ;)
Still though, I think in 3.5 it would go like this:
+6 to stat #1 = 36k
+6 to stat #2 on same item = 36k*1.5 = 54k
total = 90k
Or, for 3 stats, 144k.
Or, if you go by the example of other items...