بسم الله الرحمن الرحيم
(1) Yes, ghusl becomes wajib after seminal discharge. The sign of semen is that it is thick and one feels orgasm after it discharges and the lust is over.
(2) The fluid which comes out after the sexual excitement it does not lessen excitement rather increases it, it is called madhi. It does not make ghusl wajib, however, it breaks wudhu. The virgin unmarried woman also experience semen and madhi. And the same ruling shall apply to them. You should decide about the wetness in the light of the above mentioned fact whether it is semen of madhi and then practice accordingly.
(3) The white fluid which comes is impure and it breaks wudhu.
Allah knows Best!
Darul Uloom Deoband