What's the differences between a Resolver and a Synchro?
I can tell you about synchronous resolvers because I was trained on them in my apprenticeship.

A synchronous-resolver is probably what you refer to as a "synchro" and like you say, it has three windings (on the stator) and also a rotor winding brought out on slip rings. They are just like a synchronous motor or generator used in industry but way smaller. There use was in encoding (as you also said) but they had a great little feature in that if you wired two units back to back i.e. all the stator windings coupled back to back and ditto the rotor windings, you could apply a single phase AC voltage to both rotors (via the slip rings) and if you physically rotated one unit, the other unit would track the physical movement. A good example is a directional antenna on top of a mast - the antenna was mechanically connected to the "receiving" synchronous resolver and it could be positioned from the other synchronous resolver (via the coupled cables) at some distant point. The accuracy appeared to be phenomenal and that's not a surprise given the cost of them.

Now here's where it gets a little sticky (mainly due to my memory); the other type of unit which you refer to as a resolver is two stator coils at 90 degrees (like you said) but I don't believe it can be used in the above example. 

EDIT - this section is incorrect but I've left it in so that I can be justifiably ridiculed and tormented by my error. From memory it can do a 360 degree determination of angle but at the neutral point of the rotor (halfway between stator coils, 45 degrees) it can be flipped 180 degrees and you wouldn't know the difference. OK, end of edit.

I'm not going to say on more on this because I think you should do some research (like is being hinted in the comments) and come up with a couple of examples of units that you can buy and that have data sheets so that they can be further analysed. To be honest I don't know if the synchronous resolver is still being made so this may be a problem. All the ones I saw were pre 1970s units taken off military vessels.

A fascinating read: synchros and resolvers: -