I am one of the plastic surgeons who advocate [judicious, careful, precise, and gradually-increasing strength] steroid injections for excessive subcutaneous scar tissue after rhinoplasty (or other scar-causing operations or injuries). Your example is why the part in bold is the important part. When excessive scar tissue forms, there are only 3 options:
Live with it. The scar may soften and fade over time, but too much scar remains too much scar .
Re-operate (Hope it goes better the second time. Unfortunately, often it does not. Unpredictable. More surgery and recovery. Costly.)
Steroid injection to shrink and flatten the scar bulk and excess. Requires precise injection, starting with very low doses and strengths, waiting 6-8 weeks, then gradually increasing dose and strength until the precise beneficial effects are seen. Also requires patience. Inexpensive.
Steroid injections work by breaking bonds between collagen bundles that excessive scar is made up of. Since the exact amount and concentration of steroid necessary to produce the exact desired effect is totally unpredictable, simply shooting in a strong steroid is not in keeping with the plan I outlined in step 3 above.
Your steroid dose was (obviously, now that the effect is seen) too much or the concentration (strength) too potent. Your options are now limited to waiting to see if your body remodels the thinned tissue and improves the "dent" a bit. This is probably wishful thinking, but perhaps your body has not yet finished its excessive scar formation at 3 months post-op. If it has, the dent will probably remain much as you see it now.
Saline is very temporary and will only serve to get you out of your surgeon's office. Within an hour or two (perhaps a bit more if the needle stick induced a tiny bruise and swelling), the dent will return. Fillers (such as HA or hydroxyapatite) are temporary, but will serve as a temporary "fix." Only your own tissue (as a graft beneath the skin surface) will serve to remodel and improve contour here on a long-term basis.
It sounds as if you need damage control. Seek another ABPS-certified plastic surgeon and get another opinion, only if to serve as a second opinion. Lack of trust makes every problem worse, and every proposed "solution" suspect. You need to restore that. Answers exist; you must simply seek them out, and be prepared to pay for them. Most reputable surgeons will try to keep the financial impact of helping you to a minimum. We too have been there! Best wishes!