1. Why don't we take a tennis?

- Sounds good.

2. Why don't we take a tennis lesson?
- Sounds good.

3. Why don't we take a tennis class?
- Sounds good.

(Which dialogue is grammatical and common? Thank you.)

2 and 3 are common and correct.

1. is not.

All three dialogues are grammatical and common, but the first two phrases could be improved for clarity and understanding. When suggesting an activity like playing tennis or taking a tennis lesson/class, it is more common and natural to use the verb "play" instead of "take." Here's how you could rephrase each dialogue:

1. Why don't we play tennis? - Sounds good.
2. Why don't we take a tennis lesson? - Sounds good.
3. Why don't we take a tennis class? - Sounds good.

These revised versions are more commonly used and make it clear that you are suggesting an activity or proposing a plan.