I have a Big Boy, a Challenger, a 2-10-2, and a 2-8-8-2 Mallet and do not have any 3-foot radius turns. Are we talking HO here?
My widest turn is just under 32" inches and it is not a full 180 degrees. Most of my turns are between 22" and 28" on the main. The turns on the spurs/switching lines are as little as 16", although most are between 18" and 22". But, I'm talking HO.
A 32"r turn is one BIG, sweeping turn!!!
As far as laying out my turns, I have a compass that can layout 4' radii. Before I got it. I used the ol' string and nail trick. I Believe that, Using NMRA Standards and Best Practices, that the setback for the track from the face of a module/plywood is 2". I don't have a mainline module at hand to measure. And I always look it up to make sure. But, laying track 1" from the edgei is asking for trouble. The inevitable derailment would cause a train for a disastrous meeting with the floor with irreparable damage. And it could be quite costly to replace a Challenger should that happen.
Check with the NMRA standards and Best Practices BEFORE laying track. And it would be wise to do so BEFORE getting the supplies to build your layout.