Obviously stack them vertically at various heights. Or make walls out of them.
Then stacking them in things like 3 into a vertical triangle formation like you mentioned. Offering a low stepped set of heights that flex.
Basically any way you stack them is a good way. Angled or haphazard...its all good. Play with tire thicknesses too. Saw where people had practiced prior to the match on a regular car tire and on the day of the match a thin 4" thick spare tire with rim was used.
Here a tire strapped to another obstacle.
Saw once where one tire laid horizontal on ground while another is inset in the tire upright....tread facing downrange. Lower tire somewhat limits the vertical tire from rolling away but provided a narrow curved slick patch to rest the rifle/bag upon.
I've seen tires bolted directly together through the tread (openings facing you) suspended by frame and chain. They can wobble and bounce. Offers the open ports and the tops of the tires to rest on. Probably the most difficult tire obstacle I've encountered with the way the tires could mildly pivot or bounce under load or recoil.
We had a match where they had really big (like 5' tall) tractor tires. One was stood vertically and the inner lip was only like half to three quarter inch wide so you couldn't necessarily have your rifle span the entire gap front to back. I think one match had you shoot under the strap show here. Another on the strap.
There was a stage where you had to go back like 10' and grab a tire then shoot two targets. Go back and grab another tire, stack it on and shoot. Repeat till all 5 tires were used.
As with anything, the more thought you put into a stage...either in finding unique obstacles or creating unique but plausible scenarios will work.
Sometimes the tire is a stand in for anything of that height and size. Or it could be how would you use this thing for cover/concealment to shoot from.
On those big tractor tires I thought it would be cool to get inside one laying flat and shoot a sweep of targets...tank turret style. As a kid we had playgrounds with these big ass heavy tires stacked to play in.