5 week old has 3 small strawberry heamogenia birthmarks. Was told I could use hydrocortisone cream to help them fade by ped. Does this really help?

I don't think so. I don't think the Hydrocortisone cream will really do anything about the strawberry hemangiomas. They usually develop gradually and grow in size and redness. Then they will gradually fade and shrink. Most of them will disappear (or nearly so) by 2 years old. They only require treatment if they are causing an obstruction.
Hemangioma. I haven't seen improvement in this age group. Hemangiomas may enlarge over the first 6 months of life. Many of them then fade on their own over a period of months to years, even as long as 10 years. It is important to note the location of the hemangioma. Most locations are ok. Others, like overlying the spine, or near the eye, need to be addressed immediately. You can start with a dermatologist.
Not necessarily. These will normally increase in size and thickness naturally to about 10-12 mo, stabilize and slowly begin to fade over the next 3-4 years.Injected steroids were once given to large ones in worrisome areas like the eyelids, but I am not aware that topical hc would do anything. Some newer research suggests Propranolol may help shrink them but the med has some potential side effects.