I am still a beginner in raspberry pi world. I have a 3 B+ and a small water pump that can work with 3V to 12V, and a multimeter that I use to measure voltage.
First when I measure voltage between pins 1 & 6, I can see a clear 3.3V, then when I measure voltage between pins 3 & 6, I also see a clear 3.3V. So far so good.
Then when I connect the water pump to pins 1 & 6, it works and I can see it move water from its input to output, but when I connect the pump to pins 3 & 6, it does not work. The weird thing is when I measure voltage between pins 3 & 6 while the pump is connected, the multimeter reads only 0.4V which is not enough to power the pump. What's wrong here and why there is such inconsistency?