Master WGS
The probability here is actually a bit lower. First, the destiny knot must
select the IVs you're looking for. You want 3 IVs, which is a 3/6 chance of the destiny knot even picking HP, Speed, and Special Attack to even pass.
Then it multiplies by the % chance in each IV to select it. So start off with a 50% chance to pass HP, Speed, and Special attack, then multiply by a 50% chance to pass frogadier's HP, 50% to pass frogadier's special attack, and 50% to pass Ditto's Speed, leaving you with 1/16.