is a 20x3.0 tire too wide for trials? or is it just fine 
I think it might be a little sluggish ridding on pavement. If your frame can hold it, try it out. =p
and…
ok, well then it might not be so good for doing freestyle stuff? like one footing or whatever?
The 20x3 tires are generally for cruiser bikes. The sidewalls and construction of the tires are not for low pressure use or for bouncy jumping. If you can find a suitable tire that can work well at a suitable low pressure for getting a good bounce and compression then give it a try. If a suitable 20x3 tire can be found I think it would be good for street riding. The larger 20x3 diameter compared to the 19" trials tire size will give more clearance for the cranks when doing freestyle moves like spins or tight turns without hitting the pedals on the ground. You’re also going to need to find a suitable frame that can fit a 20x3 tire and a wide enough 20" rim.
where do you get such tires?
Various cruiser bike manufactures have them. One being 3G Bikes with the Twenty-G tyre. I have the 24" Hoggy G version. It doesn’t have the ideal low pressure riding characteristics for a true trials style tire. But the sidewalls aren’t real thin like some other cruiser bike tires, like the Dyno Fireball and similar cruiser bike tyres.
The marketing says they’re 20x3.45 but in reality they are not anywhere near that. They’re less wide than a Gazz 3" tyre. Still a fat wide tyre. Just not what the sidewall or the marketing says.
I’d like to see someone put together a quality street unicycle with the 20" Twenty-G tyre. I think it might work well for certain styles of street that have lots of freestyle moves, like the style of street that Dan Heaton does.
ihas been done by
i tink spencer
I see that Onza has a 20x2.4 tyre. Not as wide or fat as a true 19" trials tyre. Looks like it’s fairly fat though. Mod trials bikes have a 20" front wheel. I’m guessing that the Onza tyre is intended for the front wheel of a mod trials bike. They don’t jump on the front wheel so the front tyre doesn’t need to be as fat or bouncy as the rear tyre. So you’re not likely to see a fat bouncy 20" trials tyre that are as fat as the 19" trials tyres.
They do jump on the front wheel…
Yeah, but not in the same way they jump and land using the rear wheel. The front tyre doesn’t need to be as fat. And a skinnier front tyre is lighter. Lighter is something that makes trials bikers happy. They don’t want useless weight.