That 3% I believe is not considered interest, but the rate of inflation. It would, in fact, be considered without interest.
I hate to burst some balloons but a 3% interest rate is pretty much the average interest rate in the US for the last what?, 50 years? So the society lending out a loan at 3% is in line with inflation (as for a adjustable mortgage, it is just a lot easier to have a fixed rate without having to reconfigure it every time the federal reserve meets to jack up or lower rates.. The only problem I have is why the kingdom hall fund needs to be paid back at all, wasn't it all donated in the first place through the kingdom hall fund? What happens to all this money when it is paid back? The rate of growth has slowed down and now what about all this money coming in every year from these loans?