In Shakespeare's Macbeth, the Third Witch says to Banquo: "Thou shalt get kings, though thou be none" (1.3.65); if I want to quote that line and write it in the third person singular, how should I conjugate the verbs "shall" and "be" correctly?
Shall is an auxiliary verb, so its conjugation follows the rules of shall/will/can/may/should/would/could/might and is different from regular verbs. The only change from the base form is for second person singular:
I shall, thou shalt, he shall, we/you/they shall.
(And you could have figured this out for yourself; look in Shakespeare, e.g. That henceforth he shall trouble us no more.)
And the subjunctive be is the same for all subjects. So:
He shall get kings, though he be none.