I have two apple IDs becasue I spend half of the year in the U.S. and the other half in Barcelona. Can I have my three devices (MacBook Air, i-phone 6 and apple watch listed on both accounts. What is the relationship between Apple ID and i-cloud acco

An AppleID provides access to iCloud services for a particular ID. If you want to re-download items you have to use the ID used to purchase those items (unless you have connected them with Family Sharing but that does not work for IDs using different countries). If you use iCloud storage the things used by one ID are inaccessible to the other while signed in with the other ID. It's a bit like having keys to different houses and only being able to use one house at a time.

There may be ways to sort of get by with using two IDs but I presume you wish to have them associated with different country stores and that will make it even harder.

The whole system is set up around a single ID per person and that has a single country setting. If you want to not do this it will be a management issue. It's be easier on the computer since that is designed for use by multiple users (each with their own user account on the computer), but mobile devices can only be set to work with a single ID at a time.

I think that you're going to have a really complicated life if you plan to use two different Apple ID with the same devices.