Field sharing between translations of a node?
How about storing the 7 fields in a 1st content type and the 3 remaining fields in a 2nd content type. And then use the References module to connect them to each other? That way you'd only have to "translate" the content type with the 3 fields.

A variation of this could be using entities, whereas you'd then use the Entity Reference module instead. And probably also the Entity Translation module, for which quite some Community Documentation is available to get you started.

Drupal 7's new multilingual systems compilation contains an interesting set of posts about multilanguage websites ...

PS: You may also want to look at Node translation vs. Entity (field) translation for a lot of extra info/details about both variations of translations.