If a manufacturer stipulates a 12x7 prop for an engine, as an example, then what size 3 bladed prop is its equivalent? My understanding is you keep the same pitch but reduce the diameter by an inch, so in my example a 11x7 3 bladed prop is the same as a 12x7 2 bladed prop....... is this correct? Thanks in advance for your time.

Two blade at 12x7 then three blade would be 11x8 it's down 1 in diameter but up 1 in pitch

The advice I've always seen is to go down an inch in diameter and keep the same pitch. Not saying you're wrong, but I've seen the "same pitch" version a lot.

Yep, that seems to work for me, what jetmech said will suck more power from the battery than the two blader would so is not advisable unless you can provide the increased current.

Maybe it's different with electrics, but for glo engines I believe jetmech05 is correct, down one in diameter and up one in pitch. That's what I have always been told and what I have always done.

If you are flying electric, you most probably have a watt meter. Easy to prove the results, just measure current and power with each of the different props. Results will be readily seen.

From Zinger Props ... The conversion for a three and four bladed propeller is a simple tasked if you know what two bladed propeller you use on a given Engine. For example, on a three bladed propeller you would drop the diameter and keep the pitch. On a four bladed, you would drop the diameter and the pitch. For instance if you had a two bladed 24-10 propeller and you wanted a three bladed you would use a 22-10 and so on.

Anyway, here's the link to the page.
http://www.zingerpropeller.com/_2_blade_.htm

Bobby Mcgee, From Master Airscrew props, "2-blade to 3-blade conversion: decrease the diameter by 1" and increase the pitch by 1" if possible. Adjust to 2" for larger diameters (14" and up)." SO, I guess it depends on what brand of prop's instructions you want to follow. It looks like, as with so many other things in this hobby, there is more than one "right "answer. My conclusion is, "I guess it doesn't matter that much".

I don't know about electrics. I went with a 3 blade on my 170CC to slow down on down lines, and to quiet it down. I went down an inch in diameter and up in pitch I didn't notice a diff except on the down lines.

Let's be complete here. Master Airscrew also says "It is all right to keep the same pitch when going from 2 blade to 3." I think you're right that it's not a big deal. Even when not changing the number of blades, a one-inch pitch change doesn't usually affect performance much. When you go from two blades to three, a lot of times you're also changing brands and maybe blade shape, so the important thing is to see what works. The "reduce diameter by one inch and ..." is just a starting point.