I just recieved some CEN #3 glow plugs that I ordered off of Ebay. I have been using #5's till now. What's the difference? Can I use a #3 in a GST 7.7, Matrix TR or a CT5?

I use #8's in my GST, don't know what the others use

OK Pal..OS#8 is a medium hot plug I will give you a Tip: go to www.osengines.com When the page comes up go to ....OS Glow Plugs... Check out the glow plugs or how to get the correct a glow plug..
I use all 3....os#8 A3 A5 just depends on the temp of day and when you run your RC product..I have ran these glow plugs in airplanes work fine..
I also run them in my Genesis GST
Just my 2 cents worth...