I have the Brother 1500 PS I use the three hole bar for a thread guide, not for tension. I use one hole to guide the thread. The other holes are for specialty threads that need more help. A Brother tech told me this and said it wasn't addressed properly in the manual. Brother sure didn't spend any money on the manual I got with my machine. Very vague and basic.
Oh and never ever use a metal bobbin that has the open holes around it. Don't use plastic bobbins. That is true for most one stitch machines like Brother 1500 and Juki. A wrong bobbin will cause havoc and can cause damage. I know, I had my repaired twice by a dealer repairman that didn't know this and was blaming it on user error. A Juki tech said it was basic info every repairman should know.