1. I should get going.

(Is this correct? What other expressions can we use?)

2. It's about time I move along.
3. It's about the time I move along.
(Are both OK? Which one is right?)

#1 and 2 are often used.

Another one that is used is "Time to get up and at'em"

"It is time to get hot." is also used.

#3 do not use "the"

1. Yes, "I should get going" is a correct expression. It implies that you need to leave or start doing something else.

Some other expressions you can use to convey a similar meaning are:
- I should be on my way.
- I should be heading out.
- I should be getting a move on.
- I should be hitting the road.
- I should be making tracks.

2. The correct expression is "It's about time I move along." This phrase indicates that you feel it's necessary to leave or continue with your plans.

On the other hand, "It's about the time I move along" is grammatically incorrect because "the" is unnecessary in this context. The correct use would be "It's about time I move along" without the definite article "the."

Please note that the exact expression to use can vary based on the specific situation and personal preference.