I wrote a check for $300, but the bank posted it as $3,000. The bank told me to get the money from the other party. Is this correct?

No. This appears to be a check altering/posting error. The bank must correct it. You should notify your bank so it can investigate the error. The bank may require you to complete an affidavit.

The time period for this prompt notification may vary by bank and State; if a check was altered, you may have up to one year to assert one alteration. Generally, for multiple alterations, banks require you to notify them of any error(s) within 30 days after you receive the account statement.

Your Deposit Account Agreement specifies your bank's specific time requirement. Review that agreement, then contact the bank directly regarding any alleged error.