Honestly, I wouldn't worry about it. The kids are going to have more fun running around your backyard and climbing on the play structure than they will sitting in a stroller anyways. YOU may go stir-crazy, though. I have a triple stroller too, and 4 little ones 2.5, 2, 22 months and 18 months. The second-oldest walks, because I've had him over a year and I trust him not to bolt. However, he can't go too far, and is having a hard time remembering to keep his feet out of the way of the stroller wheels. The youngest doesn't come on Thursdays, so Thursdays he gets a ride, but every other day he just WANTS a ride, and can't have one. And he's the best candidate for walking, too. I'm in the market for a quad, fingers crossed I find one soon! Would it be worth it to you to invest in a larger stroller?

You could also look into different outdoor toys to keep it interesting. In the summer I did a "sprinkler day" every Friday and turned the little slide into a water slide with the hose and brought out the water table. If you have the storage space, you could bring out different outdoor toys (ride-on toys, bats and balls, etc) on different days so it feels new to them even though they're in the same space?