it would be alright if you had two of each, but 3 different sizes? Maybe for a spare... The 3 3/4 may work in the rear, but not in the front. Just be sure that the two front are the same, the rear is not as important, but I might consider buying some new wheels, 3.5" BS wagons can be had for pretty cheap new or used.
Like CruisinGA said, I'd be sure the guys up front were matched just to prevent headaches like pulling to one side. The trouble you run into then is how to rotate your tires for even wear. A complete set wouldn't be too much $$$ if from ebay or the like.