I recently bought a Raspberry Pi 3 and forgot to buy the power supply. Could I use a phone charger (1.5v, 0.75amp) to power the Raspberry Pi 3?
I believe the minimum is a 5v 2.0 amp power supply however most kits provide a 5v 2.5 amp power supply. Part of this depends on what you are actually doing with the Pi and what devices you have turned off as part of saving power.
See this posting about power use Raspberry Pi 3 vs Pi 2 power consumption and heat dissipation
See this FAQ from Raspberry Pi https://www.raspberrypi.org/help/faqs/#powerReqs which contains a table of recommended power supply requirements. For the Raspberry Pi 3 Model B the recommendation is
Recommended PSU current capacity - 2.5 amps
Maximum total USB peripheral current draw - 1.2 amps
Typical bare-board active current consumption - 400 milliamps
There is this article that provides a fairly indepth look at power requirements of the Raspberry Pi 3. http://raspi.tv/2016/how-much-power-does-raspberry-pi3b-use-how-fast-is-it-compared-to-pi2b
.75 amp will not be a good idea and probably won't work reliably. I remember reading that phone charger seems to be something that many people first think of but a phone charger is not designed for the application of powering a Raspberry Pi and it is not a good idea to use one.
Remember that the entire package of Raspberry Pi and its electronics as well as the USB ports for keyboard, mouse and HDMI, WiFi, Bluetooth, and Broadcom GPIO board (if you are doing electronics experiments with your Pi) need power. Power consumption by the CPU will vary depending on the processing load and the number of cores in use.