Although the episode "Friends and Neighbors" is officially an episode of The Ropers, it also aired as a regular episode of Three's Company. In that episode, when Mrs. Roper finally comes back home to the party after her disastrous trip to her art class, you can clearly see that she is wearing trousers instead of her familiar long gown.

Were there other episodes in which Mrs. Roper wore pants?

I never saw her in pants.

You can see her in pants on page 17 in the tc book. It's the 2nd pilot.

i seen her in pants. i forget what the episode was though. it was in the first season i think...

The one when Chrissy was typing the diary.

I noticed another instance of Mrs. Roper wearing pants after watching tonight's episode of "The Party's Over". Mrs. Roper is wearing red pants after returning to Stanley.

I don't think I've ever seen her wear pants.