At my company, payday is posted as being paid every 2 weeks, in this case every
2nd Monday. If it is posted as such, is it legal to not receive your paycheck 3 days later?

What do you mean as "posted" advertised in paper ? Or do you mean posted in the accounting books? Are checks done by a third party outside firm ?

Are checks always paid on the same time frame ?

Posted as by law, they are posted on posters from the state of Ca. but they include, for example,
1. where you go to the doctor if hurt on the job
2. when time cards are to be turned in and pay date, in this case every 14 days on a Saturday and paid on the following Monday.
3. where you can receive information concerning your rights as an employee.
No they are not done by a third party, and yes they are paid normally on that Monday. I was informed, and I'am on salary, that the checks won't be ready until
The following Thursday.

If there is a posting in the employee handbook or on notices in an employee area that paydays is x, then a company is in violation if they do not meet the date.