Where do you live?

Was all the grass installed (seed or sod) at the same time?

Can you take pictures on a cloudy day? Grass pics in the bright sunlight are very hard to get the details from.

I'd like to see a picture of the bright green grass and the deep green grass from a height of about 6 inches. Those look like different kinds of grass. I'm thinking crabgrass or St Augustine.

I'm wondering if the wheels on your mower are all set at the same height.

That or mowing ruts?

The OP said it was sod. That certainly looks like a roll of sod that was different somehow. My question is were these sections always different or did this show up after three years?

tj

it could be a different soil type or a different type of grass

Thanks y'all - our house is in Texas and the lawn came from sod. I'm going to check the mower again to make sure and try @Yardvaark's recommendation to do a test.