Roger
For openers, look at the AMA rule book.  It requires model weight 8 to 15 lbs multi strand 2 lines use .027 dia. lines. or for 3 lines use single strand .016 lines.  Pull test to 8G/55 lbs min 80 lbs max.  The chart in 2005 / 2006 did not show a line size for 3 line multi strand but you would be acceptable using 3 of the .027 size lines per next line in chart.
 
For a 15 to 20 lb model it calls for multi strand .027 for 3 lines, pull test 5G, 80 lb minimum.

I have flown many times on the first set of .027 stranded lines I started with in 1982.  I use two lines and electronics. 

Clancy