Hi Michael.
I don't know if it'll be an expert advice, but I'll do my best.
All this models belongs to the verb "to be" in a conditional way, which "fosse" is past, "for" is present and, “seja” is??? Future? Not necessarily… I’ll try explain it better bellow
First, see the below examples with “fosse” and “for”, they are easier to understand, I guess...
Examples with "fosse" (past):
Se ela fosse mais bonita, eu namorava ela - If she was more beautiful, I would've dated her.
Se eu fosse mais forte, eu teria salvo ele - If I was stronger, I would've saved him.
Examples with "for" (present):
Se eu for inteligente, eu resolvo esse problema - If I am intelligent, I can solve this puzzle.
Se você for rápido o suficiente, você pega ele - If you are fast enough, you catch him
However “seja” it’s a bit tricky and it has many equivalents in English, but overall I see it in three main uses… as 1) a conditional; as 2) a willing or command to someone to be something; or 3) “in other words"… see the below examples.
1) conditional:
Caso ainda seja cedo, volte para casa - In case it’s still early, come back home
Caso você seja menor de idade, não dirija - In case you’re under age, don’t drive.
Seja ele bom ou ruim, mate-o!! - Whatever he is good or bad, take him away!!
2) Willing or command:
Seja homem! = Be a man!
Trabalhe e seja feliz = Work and be happy
Espero que você seja uma boa pessoa - I hope you are a good person
3) In other word:
Ela é legal, ou seja, namore ela!! - She is nice, in other words, date her!!
Ele não tem as habilidades requeridas, em outras palavras, não contrate-o - He doesn’t have the require skills, in other words, don’t hire him
I hope I could be helpful and any other questions, let me know.