Customer A deposits $6300 into his personal account
Customer A deposits $6000 into his DBA account
Customer A deposits $5300 into a Corporate account

Part 1
Customer A (2.a and 3)
Customer A (2.b and 3) Unsure about this needing 3
Corporation (2.c)

Am I thinking correctly?

Any thoughts?

I agree with you and I would not check multiple transactions on customer A's second part I.

Customer A and the sole proprietorship are one and the same person. Both the first and second deposits are by Customer A on his own behalf (2a and 3 checked), with both account numbers and $12,300 shown.

Customer A also made the deposit to the corporate account. So his second Part I would have 2b and 3 checked.

And, of course, there's the Part I for the corporation, with 2c checked.

Box 3 on both Part I sections for Customer A because he's a conductor or owner on more than one transaction that is part of the CTR.

"Filers should report Item 3 “Multiple transactions” when a CTR includes more than one transaction conducted by and/or for the same Part I person on the same day. Item 3 only applies to a Part I person, owner or transactor, involved in more than one transaction in a single CTR."