The wheel widths on every Yanmar I have seen with conventional agricultural-type rims are adjustable, to a certain degree. An adjustable center disk gives perhaps 8" of adjustment overall, while some smaller models use sliding, pinned hubs on the axle. The 4 wheel drive units don't have adjustable front ends, though, and some 2 wheel drive machines (even within the same model) lack the ability to change the axle spacing in front, too.
When you say lay 36" rows, are you meaning to allow the tractor to straddle them, or to fit between the rows? Assuming you mean to straddle them for cultivation purposes, you should be fine with the 2310 or any other non-tiny version. I don't know that you would be able to run in the same tire tracks both directions up and down your rows at 3' spacing, but this shouldn't be problematic for most crops.
What are you using to tend your garden now? What crops are you planting? What equipment or implements do you plan to use?
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