ok i drafted 3 guys in rule 5, and i have salary cap and 40 man room for them, but i dont want them on my big league roster if i can avoid it

can i assign 1 to ML roster, then demote him to aaa? -- do this 1 at a time and get them all in aaa eventually -- all have minor league moves still available

or is there another way to keep them on my team without putting them on big league roster for the entire season?

i know i can hold them in "assignment limbo" for some period of time, but i think this elapses quicker than a normal WW limbo guy would

No demotion, since they are Rule 5. You can assign one, then begin the offer-back process with him, and if no one else wants him and the original owner declines his return, demote him. And do the same with others. If you don't want to do that, they have to stay in majors all season.