Is it normal for a baby to start walking 3 weeks ago but walks on tiptoes?

Yes. Toe walking is a normal stage in some toddlers and is common off & on to the age of 3. Most do if for a while & progress to regular teal toe walking over time.
Toe-walking can be. A typical variant for 3 months after a toddler can walk across a room after that, a search for a neurological or neurodeveopmental cause is needed. If he is 100% toe-walking now, it may be wise to see your pediatrician for comprehensive developmental, neurological & physical exams for diagnosis & treatment. Assessment & physical therapy by an early intervention program can also help, if needed.