one of the adversaries nemesis (pag. 412) has force rating 3, as is possible if the talent tree exiled force only gives 1 rank apart from giving specialization?

#1: NPCs are not subject to the same character building rules as PCs, unlike Saga, although...

#2: Age of Rebellion introduces a second Force specialization that also offers a Force Rating +1 talent, so if you had to recreate this character and were allowed to use both books with which to do so, you'd buy both specializations and then buy the (ranked) talent in both talent trees.

The main point with NPCs is that unlike the d20 system, they aren't bound by the same rules as the PCs when it comes to assigning them Talents, Skill ranks, or any other trait. NPC design in this game really boils down to "what looks right to make this NPC an interesting part of the encounter?" be that encounter centered on combat or social interaction.

The Forsaken Jedi also "breaks the rules" in that as their powers are written (and this was confirmed by Jay Little and Sam Stewart), they only need to spend a single Force Point to activate not only the basic effect of their powers, but also any upgrades listed' so yes, with a single Force Point the Forsaken Jedi can use Move to hurl up to two Silhouette 1 objects out to medium range, where by contrast a PC Force-user would need 4 Force Points to do the same (1 for the base power, 1 for the Magnitude Upgrade, 1 for the Strength Upgrade, and 1 for the Range Upgrade).

For PCs to get to Force Rating 3, they'd need to start with either the Force Sensitive Exile or the Force Sensitive Emergent (Age of Rebellion's universal Force-sensitive specialization) to get Force Rating 1, and then purchase the Force Rating talent (which is a Ranked talent same as Dedication) as Chortles noted in each specialization tree. Once Force & Destiny comes out, the option will undoubtedly exist to go beyond Force Rating 3, but it's bound to be expensive since you'd have to purchase even more specializations, which are likely to be non-career specs if you've been playing a Force-user since Edge of the Empire or Age of Rebellion came out.