Training on restraints for us was done by our therapist. Your psychiatrist might also be able to. You need to make sure that *both* of you are trained and that it is documented in your son's chart that training was done.
I think you actually need to have 2 safety plans - 1 for which both of you are there and 1 for when only 1 parent is present. Most of the time around here, I was the only parent in the home. Up until thank you was about 9, the safety plan consisted of my youngest son taking my daughter, going into my bedroom (which had a TV and toys just for these occasions), and locking the bedroom door. His cue was simply me saying "Wee, get Diva and go." I would roll oldest into kitchen, since most of the fun and games happened in the living room or his and thank you's room. I would keep phone nearby in case things got insanely out of control, and I would restrain thank you. If he didn't chill out in a reasonable amt of time (10-15 mins), or if he was in one his mega-rages where it was impossible to restrain him, I would call 911 and request an ambulance to transport a mentally ill child. I *always* made sure to tell them it was a mentally ill child. I'd usually end up with- 2 or 3 police cars, the ambulance, and the head of EMT services for our village here. They got to know us pretty well very quickly, so they knew what to expect.
I would never try to transport a raging kid to a hospital by myself, or even with husband in the car. We had 1 incident where the meltdown started in the car with both husband and I in it. I ended up having to pull over because it was simply impossible for husband to manage thank you and protect the other kids by himself (and he's 6'5", 275 pounds).
For the few months thank you lived at home after age 9 (3 months at 12 and then 3 more months at 14), the safety plan changed. He was bigger than me and there was no way either husband or I could restrain him safely. Plan was call 911 and request ambulance, period.
I know 3 seems really young, but I would work with him on removing himself in case it's only 1 parent in the home during a rage. Diva was about 2-1/2 when thank you left for his first Residential Treatment Center (RTC), but even at that young age, she understood enough to go to my bedroom and at least close the door when I told her to go and thank you was raging. Plus, the flying objects were pretty scary, so she did get it.
I think which option you choose (calling therapist/psychiatrist/crisis line/911) has to depend on how unsafe the situation is. If you are able to contain difficult child, then the first 3 options are reasonable. If he's continuing to escalate or trying to engage in unsafe behaviors (knives and jumping out windows were the biggies when thank you was younger - when he was older he started taking swings at me and that was equally unsafe in my book), then you need to call 911, in my humble opinion.
Hang in there.