3.5 cc = 3.5 mL
The concentration is 10 mg/mL, meaning each milliliter contains 10 mg of dexamethasone.
Since 3.5 mL was injected, the total dose is:
3.5 mL × 10 mg/mL = 35 mg injected
I'm not sure which HCPC you're using, but both of these are billed one unit per 1 mg anyhow:
J1094 – Injection, dexamethasone acetate, 1 mg
J1100 – Injection, dexamethasone sodium phosphate, 1 mg
So, the total billable units would be 35. One unit for each milligram injected.