I don't know why people assume a four-cylinder would be smoother than a three-cylinder, but a lot of people do -- perhaps it's the "more is better" attitude, or just the intuitive thought that more pistons and firing pulses are smoother. But an inline-four has some inherent balance problems -- for one, pistons move in pairs. This leads to a secondary imbalance that can be quite bad. Auto manufacturers deal with this on larger inline-fours by installing balance shafts, which add counter-rotating weights to counteract the reciprocating mass.
There's some information you can read here:
Engine balance - Wikipedia
Couple snips:
A lot of that is engineering gibberish and not as cleanly written as I'd like, but frankly I'm an engineer and I'd probably do a worse job if I tried to explain it! I don't want to hijack the OP's thread further, but there is a ton of information on the internet if anyone wants to read further. Bottom line to me is that there are definite reasons to choose certain engine configurations, but nobody should conclude that a four-cylinder tractor engine is smoother or more desirable than a three-cylinder, all else equal. It's mainly size/packaging constraints that drive us into fours as tractor size and HP goes up, not because they are better engines.
Anyone who has run the very common B3200/L3200 four/three engines of the same displacement and HP has discovered this first hand. The difference is so significant, it's hard to miss. It's the kind of thing that is so much easier to experience/realize in person than to explain. Sort of a "seat of the pants" engineering analysis.
