Best way to flat small scars? 3mm scars (Photo)

At least from the photos provided, these appear to be hypertrophic scars. If so, when it comes to treatment, keeping it simple, as with the solution to many problem, is so often the best route. Intralesional steroids in the appropriate concentrations and amounts would be a good start. In more resistant sites, additional flattening of the elevations can be achieved with the use of light electrocautery or the Ellman Surgitron. So many of the other expensive treatment modalities you mention in your question, while heavily hyped on social media and heavily marketed unfortunately lack the hard science to back up their use. So, "buyer beware" when offered these approaches.
You would be wise to consult with a board certified cosmetic dermatologist scar treatment specialist and best of luck.

Our treatments use combination therapy to improve the epidermis and dermis for most deep scars. The options available for acne scars depend on the character of your scars, in terms of topography and contour. We see atrophic and hypertrophic acne scars, as well as scars that have hypo and hyperpigmentation. We begin each scar evaluation with an assessment of scar quality and subtype. Our main concern is rebuilding collagen and that is why we use a HIDEF protocol that includes combination therapy. Morpheus 8, scar subcision, fibrous release, fractional laser, CO2 laser, Thulium laser, PDL Vbeam laser, and fractional resurfacing can be used in addition to TCA Cross and chemical peels to further improve skin. Patients should start Melarase AM and Melarase PM for active hyperpigmentation. Always begin with a consultation to decide whether early subcision should be started.

Best,
Dr. Karamanoukian
Realself100 Surgeon