Retired insurance agent nonemployee compensation. where do I report this? This amount is shown in box #3

Report this 1099-MISC in the Income from 1099-MISC interview, found under Federal Taxes > Wages and Income > Other Common Income.

Since you are retired, the income is not related to your "main job". 

Since these are (likely) residual commissions, you aren't providing any current services or engaged in any profit-seeking activities.

You probably can't say, with certainty, if you will receive these in future years.

Answering the 1099-MISC interviews with the above in mind will result in the 1099-MISC getting reported on Line 21 of your Form 1040 (Other Income)

so when a retired insurance agent still receives commissions, is that considered a passive activity?

No. Commission is considered earned Income. 

For more information check MinhT1 response.