[[There is no such thing is a “normal earth day” as a constant period of time. The day is measurably longer today than is was even 100 years ago die to things like tidal acceleration.]]
Psssst- a ‘day’ is still a cycle of one dark and light passing- whether it’s 18 hours, or 24 hours, God taleked about a day and night cycle in creation- a normal length day is dawn till next dawn
So if God created the sun on the fourth day, how were the first three days measured?
Considering that the Sun wasn’t created until the 4th day, you are correct, the first three days were definitely not solar days. If you read Bereshis (Genesis) with more detail, he created the first day by separating the light from the darkness. His light (Shekinah Glory) filled the Universe and was the source of all light.