I never use #3, that's a retail (take-home) item that I either sell or they buy elsewhere depending on their preference.
For olaplex treatment I have a 2 step process:
First I mix 15ml of #1 into 100ml water (115ml total) and massage that into their wet hair for 5-10min. Then, without rinsing, I put on #2 (undiluted) and massage that as I would a conditioner, let it sit for 10-20 min with or without heat.
You don't have to use the whole 115ml of #1 mixture, and the leftover will keep for up to 3 weeks to use on other clients if you keep it in an airtight bottle.
I offer this as a $25 add-on service to color or a $25 stand-alone conditioning treatment.
For the add-on to color - I do this immediately after rinsing the color and before the shampoo to repair any bonds broken during the color and to seal the color into the hair. Then I shampoo and condition as usual.
For the stand-alone conditioning treatment - I shampoo first, then do this instead of conditioner, and for added effect I will do step #2 under heat.
The two offerings have different functions (repair damage/seal color vs. just deep conditioning).
Our stores offer the Olaplex in a traveling stylist kit which has one small #1 and two small #2s. This size lasts me several months and only costs about $50 total. I found it to be a good entry point to allow me options and test whether my clients are willing to pay for these. (they are.)