Spain for 10 days

Yes and no. However you may or may not think that is enough after consulting guidebooks pinpointing your interests in what you want to see and do in those 3 cities over 10 days. For real. Most here giving advice have been to Spain more than once after trying to spend 10 nights in the 3 cities you want to go to.

As the previous poster indicated, it really boils down to what you want to see and do. Assuming you have 10 full days on the ground - maybe yes and maybe no. Bear in mind that every change of location takes anywhere from 1/2 to 3/4 of a day.

How many nights do you have in Spain?

Although Madrid and Barcelona are connected by very fast AVE trains, the trip to San Sebastian takes considerably longer. As one who preferred Bilbao to the much more touristy San Sebastian, I think it's a shame to travel all the way to the Basque Country just to see San Sebastian.

Madrid is blessed with a number of fabulous day-trips in addition to some very important art museums. Personally, I find Toledo a much more interesting city than Madrid if you don't wish to spend a great deal of time in art museums.

In addition to its share of museums and an interesting medieval district, Barcelona has key modernista buildings, many of which require pre-purchased, timed, tickets to avoid standing in very long lines and possibly facing sell-outs. The need for guessing in advance how much time to allow at each of those sights makes efficient itinerary-building a challenge and short visits there inadvisable. [edited to finish the final sentence]

Time of year also matters: I would stay away from San Sebastian between Nov 1 and May 1 (high risk of rain), and I would minimize the time spent in Madrid between June 15 and Sep 15 (High risk of 100+ weather).

You are pushing your luck. I would plan on 4 nights minimum in Madrid and Barcelona each.

Seville is a must do place in Spain as well.

Seville is undoubtedly one of the highlights of Spain, and personally I'd choose it over Madrid or San Sebastian. However, Paris is one of the highlights of Europe, and I don't see it on this itinerary, either. It makes sense to minimize the geographical extent of a short trip, and if there's interest in San Sebastian, Seville doesn't fit well here. It is fodder for a later trip, along with Cordoba and Granada. And if this is a summer trip, omitting Seville and the rest of Andalucia would be a very smart decision!