Stobaeus gives some definitions, that eros is driven by external beauty and aims at friendship, pothos is the desire for a lover who is absent, imeros the desire for a friend who is absent. I can’t say that I agree with these very much. Check them by reading texts on love, as is Plato’s Symposium and Phaedrus. In my opinion, imeros is the emotional result/content of eros, a rather internal condition than a direction toward the beloved person, while pothos is the intense impulse and turn to a person, the expression of eros. Schematically speaking of course. Read Plato and make your definitions.
George I think you made a mistake. Pothos is the desire for a friend who is absent and imeros is the desire for a lover. The verb potheo-potho means "to miss". This word appears in Homer too, when Patroclos is dead and Achilles misses him. Homer uses this verb and unfortunately this is why the "professors" are certain that the relationship between Patroclos and Achilles was something more than friendship.
Well, Plato says, ἔρως δέ, ὅτι εἰσρεῖ ἔξωθεν (PL. Crat. 420a), in fact it is not exclusive ox sexual desire, for instance, αὐτὰρ ἐπεὶ πόσιος καὶ ἐδητύος ἐξ ἔρον ἕντος (IL. I, 469) (when they had fulfilled their desire of drinking and eating). For the oder two, always Plato says; καὶ μὴν πόθος αὖ καλεῖται σημαίνων οὐ τοῦ παρόντος εἶναι (ἱμέρον τε καὶ ρεύματος) ἀλλὰ τοῦ ἄλλοθι που ὄντος καὶ ἀπόντος, ὅθεν πόθος ἐπωνόμασται ὃ τότε, ὅταν παρῇ οὗ τις ἐφίετο, ἵμερος ἐκαλεῖτο· ἀπογενομένου δὲ ὁ αὐτὸς οὗτος πόθος ἐκλήθη. (PL. Crat. 420a) (pothos, instead is called this way to signify that it is a wish of something that is not present, but is far away, and it is for this reason that it has been called pothos, then, when that of which somebody had a desire is present, it is called imeros.). In effect, also this is not exclusively a sexual desire, for instance, ἡνιόχοιο πόθος. (IL. XVII, 439) (wish of the horse-driver, that is the wish of the horses for their driver, Automedonte), and, τῷ δ'ἄρα πατρὸς ὑφ'ἵμερον ὦρσε γόοιο, (OD. IV, 113).