As per the docs:
| user_id | string | Returns streams broadcast by one or more specified user IDs. You can specify up to 100 IDs. |
|---|---|---|
| user_login | string | Returns streams broadcast by one or more specified user login names. You can specify up to 100 names. |
So
https://api.twitch.tv/helix/streams?user_login=bob&user_login=foo&user_login=fred