Translation:My uncle watches three Spanish channels.
You usually don't see something on tv, you watch something (systematically, in this case). I guess that's why it's not accepted. ^.^ You could use see in other cases, like
I saw it on the news. - Το είδα στις ειδήσεις. (Saw it at a certain moment, not actually watched it.)
this may be confusing, but it's also interesting. We automatically know that he isn't watching three channels at once, so it's a "present continuous" showing habit of watching. But if you're learning to speak Greek you don't need to know that, you just need to know "βλεπει" and "ειδα". Context will tell.