No, you must use a street address for the company's principal place of business. This is shown in box 3 a. You cannot use a P.O. Box address. The mailing address in 3 b can be a post office box. Principal place of business is different from the agent from the agent for service of process, which can be a registered agent.
Mr. DeWitt gives the accurate answer. Many clients use the address of the attorney who normally also serves as the agent for service of process.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
If "virtual mailing address" means a virtual office that has a street address (Regus, UPS Store, etc), rather than a PO Box, then yes, you can use that as your LLC's address.
As concerns the agent for service of process, you just have to choose an individual or entity as the agent - the street address of the agent (if not a corporation) flows directly from that choice.
This information does not constitute legal advice and does not establish an attorney-client relationship.