My question is,
When Imam Mahdi atfs attains martyrdom and Imam Husain a.s will come back (this will be his Rajat) and give ghusl and kafan and bury him.

Will other Imams a.s also come after Imam Husain a.s?

Will there be a Rajat again for Imam Mahdi atfs since every Masoom will come back in Rajat?

Yes our hadiths indicate that all other Imams will be resurrected by Allah later in the Raj’ah.
As for Imam Mahdi, I haven’t seen a hadith that states he will be come back again during the Raj’ah. Yes, there is a hadith about Imam Ali returning numerous times during the Raj’a.