I would find a 2 blade that's been properly "clocked" and identify the prop blade that's at 2:00 when the engine is coming up on it's compression stroke. Then lay that blade over one of the 3 prop blades and reference the already drilled 2 blade prop holes to mark the 3 blade.

UNLESS this were a DLE 30-35. With one of those I would go with a single bolt prop adapter and forget drilling and/or indexing.