3 weeks in Portugal:late September or late October?

Go before. The weather will be much better plus late October is half term school holidays in many parts, so places will be busier and accommodation more expensive and overrun with children.

I'd certainly go in late September rather than in late October. The pros: better chance of fair weather. Cons: More people there (with good reason).

3 weeks means last 10 days of Sept and first 10 of October, or pushing into November.

My trip was this year, mid-May to early June. There were few crowds, except for Sintra where I began, and weather was pleasant, not too hot and little rain. Just looking at the calendar, Sept-Oct would be similar. OTOH later in the year might mean lower prices and no crowds, maybe not even in Sintra. There may be fewer walking tours and day tours offered. You are more likely to get fall colors in the Douro Valley later in the year. If you are planning to go to the south (Algarve) early would be better, sounds like much of the area closes up for the winter.

Thanks for the help. I think we'll go in the three weeks before