First, are Spanish and Italian mutually intelligible when an Italian talk to a Spaniard ?
Second, what about the mutual intelligibility between Spanish and Portuguese ?
Third, would the mutual intelligibility between Spanish and Portuguese be probably much more than that between Spanish and Italian to a large extent ?
Finally, which language (of these 3) might be mostly worth learning ? I mean that, If somebody learned one of them, this language must have the largest mutual intelligibility with either of the other 2.

Question

First, are Spanish and Italian mutually intelligible when an Italian talk to a Spaniard ? 
Second, what about the mutual intelligibility between Spanish and Portuguese ?
Third, would the mutual intelligibility between Spanish and Portuguese be probably much more than that between Spanish and Italian to a large extent ?   
Finally, which language (of these 3) might be mostly worth learning ? I mean that, If somebody learned one of them, this language must have the largest mutual intelligibility with either of the other 2.

Answer

First: No.
Second: More than italian - spanish, but it's not like they can understand eachother very well.
Third: Yes.

I'd go with Spanish based on the amount of people who speak it. Picking a language solely based on mutual intelligibility is probably a bad thing to do, so just pick the one you're most likely going to use.

Answer

1) Just for curiosity, when i use to talk in my local dialect (south of italy, used in sicily island) spanish persons (from catalogna above all) understand me clearly, maybe more if i can use italian :) Funny, or not? However, it's not so complex to understand each other (SPA-ITA), i'll try to contact some users that use POR just to see if it's easier, never talked witha portoguese!
2-3 dunno....
last) Italian language is very very difficult and complex, but is beautiful and more useful than you could imagine!

Answer

I agree with Harregarre.

I learnt Spanish first. If you are very interested in languages, you can learn the
other 2 languages too, just for fun. It will be easy, if you don't mix 'em up. =)

Good luck,
Saïd.