In humans it is normal for the sleep cycle to be about 90 minutes. So over that 90 minutes (give or take) we will alternate between light sleep and heavy sleep. The cycle time is highly varied even for one person as it depends on a number of factors. So ninety minutes is just a nominal value.
Typically the heavy sleep comes right after falling asleep and the light period comes toward the end of the ninety minutes. Although it's not always, we frequently awaken during the light period of the cycle. The slightest noise or light, or the urge to go pee, can wake us up during the light sleep period.
So it's not surprising that you awake after about three hours as that's the end of the second 90 minute sleep cycle. It's also normal to sleep through more than one cycle because nothing woke you during the light sleep period.
For me, for example, I go to bed at 1200 and wake up around 0300 and again at 0600 regularly. There are exceptions, but that's my normal sleep routine. Clearly my sleep period is 90 minutes long.