Just to add a few points ...
S9's need a 240v supply, so you won't run them on 120v 15 amp circuits.
A 240v 30 amp line should support 3 S9's, but here's what you want to keep in mind going forward
1. Each S9 draws about 5.8 amps (based on published wattage figures). The PDUs should only add a little (you'll have to check the specs). Most of what goes in one end comes out the other. The only addition is a fan and the loss from efficiency, so i think we can estimate about 6 amps per PDU+S9 pair. That makes 18 amps for 3 miners.
2. You cannot run continuously at the max load of a Panel, breaker, or branch circuit (line + outlet). When planning for continuous draw you need to increase the maximum load capacity. There is an exact formula for code, but an easy rule of thumb is if your system capacity is twice your continuous draw, you'll be fine. So your 30 amp line can power your 18amp miner draw - but you need to check your overall continuous draw on the panel itself. Most household loads are not continuous , but A/C can be, so add your miner power draw to your A/C draw (for your house) to get an idea of you total continuous draw.
Don't forget to factor in A/C for your miners. You'll need to cool them in summer months and they will run continuously .
You can repurpose the 30 amp heater sockets (if you don't need them) but i would at least get new breakers. Old breakers can get wonky especially if you start putting a lot of extra load on them. They don't cost much.
Can I add additional breakers?
.... sometimes. You should ask some one with more knowledge how much is ok for your situation.
A sub panel: No harm in doing that, but if you are only going to run a single 30 amp line to it there is no need for it. Before you go this route figure out if you have enough power available from your existing main panel to run multiple 240 branch circuits off a sub panel.
Upgrading you panel:
This can be more expensive than you might think. Many houses have a 200 amp panel. Replacing with another 200 amp panel is fairly straight forward and the panels on cost about $80, but if you upgrade to a higher capacity you could see a lot of additional charges. I've been there. For one, electricians seem to have a minimum fee they will charge for changing a panel that can be upwards of $1000. Don't ask me why, I think it's a pretty straight forward job for an electrician. Maybe they just know the average DIYer can't/won't do it themselves.
Then you may be required to pull a permit from your local authority for an upgrade. This can run upwards of $150. Additionally, the riser pipe and maybe the weather cap will need to change. In my area they will not allow a PVC riser tube (from the panel up to the weather cap) on a 400 amp drop. It must be steel. etc etc etc
Then the upgraded panel will cost more. A 400 amp panel runs about $250, and 800 amp panel can cost $1000 or more.
If you want to grow past 3 miners, I would consider adding a second panel rather than upgrading the current one. I don't know your skills, but i could mount the panel, run the riser, install the breakers, run the branch circuits and outlets myself and then just call an electrician or the power company to install the drop and the meter. You'll have two electric bills, but you might be able to get a commercial rate on the second account and save money. And you'll save thousands on the installation.
If you are converting your garage and you can put your panel on the outside wall of the garage, then you'll have short lines to pull for circuits. You don't need all this extra power for the house anyway.
There are some simple books on electrical basics at the home improvement stores. They won't make you an electrician, but you'll at least be able to pull your own branch circuits and with a little home work install a panel. It's not rocket science. Just don't buy the cheapest materials you can find (you want them to handle continuous heavy loads) and make sure you check the codes in your area.