I want to have a three oval layout, should I use 15 inch radius for the inner oval and 22 inch radius for the outer oval
Are you talking a 4x8 layout, or something bigger??
Len
A better idea would be to make an approximately 20 inch radius curve that will fit between the 19 and 22. You can do this by alternating 22 and 18 sections. for a 180 degree turn you'd use 22-18-22-18-22-18-22. You might need to add a short 1" straight to the middle of the outer track's 22" radius curve to keep trains from sideswiping on the outer two tracks.
You should be able to use 22"R, 18"R, and 15" radius curved track pieces. However - assuming your in HO - you will find that some (perhaps many) engines and cars will have trouble navigating the 15"R curves. Although 18"R has been the 'minimum' minimum range in HO for some time, many manufacturers are now recommending running their equipment on 22"R or larger curves.
You might find one loop of 22"R track with room for spur tracks to industries and for buildings and scenery might prove to be more appealing in the long run.
I run an On30 2-6-0 and passenger on mine with 15" radius on the inside lap. As long as you run too large of a steam loco, you should be ok.