My way of selecting a 3-bladed prop for an engine is as follows: Assuming a proper sized 2-bladed prop, plus its RPM is known for the engine, enter those data in any decent propcalculator, and note down the power generated.

Now search that same propcalculator for a 2-bladed prop that absorbs as close as possible to 2/3rd of that power at the same RPM. That is the size of 3-bladed prop you are looking for.

If APC is the brand to go for, indeed an 11 x 6 would be very close to an APC 2-bladed 12 x 6. Slightly heavier.
However, a Master Airscrew for example, would need to be 11 x 7 in order to be equivalent to a 2-bladed APC 12 x 6.
When going from 2-bladed to 3 bladed, it is always advisable to select a prop with a bit more pitch, the higher velocity of the propwash compensates a bit for the narrower diameter of that propwash, and flight performance will be more similar. At least, that is my experience.