1 wheel at the front, 2 at the back (e.g Reliant Robin) - the engine mass is effectively balanced on 1 wheel and the driver is sitting outside the roll axis (lines between the front wheel & the back wheels) and the arrangement is intrinsically unstable. Hence a heavy person such as J Clarkson, with a bit of help from a downhill left hand turn and probably an abrupt steering input and a dab of brake to further shift weight to the front can overturn this configuration relatively easily (as shown on Top Gear the other week although one must take any "reality" shown on Top Gear with a pinch of salt).
2 wheels at the front, 1 at the back (e.g. Morgan 3 wheeler) the engine is centred between the 2 front wheels and the driver is more or less sitting on the roll axis so his weight is at least not assisting the overturning moment. Hence it's a lot more stable (they used to race Morgans at Brooklands)
And as you may have noticed, single engined aircraft with a single nosewheel are relatively unstable on the ground in roll
Probably more so than a tail dragger