I am making a simple heater with the help of ESP8266 wemos d1 mini. Heaters will use 12V DC power suply and ESP8266 must turn the heater on and off with the help of 5V DC relax. I want to avoid the use of anything else then ESP8366 and relay. I will need 2 such relays. So the question is, can i control 5V output on ESP8266 ? Or do i need to find a 3V DC relay and use that ? If the latter is the case, can i get some ebay link to 3V DC relay ?
Voltage is only part of what a relay uses. Check the current requirements for the relay coil/input as well.
For example the common result for "3V relay arduino" will produce results for relay MODULEs that contain something akin to a SRD-03VDC-SL-C relay (labeled on the relay itself). Spec for the RELAY (separate search by relay part no) yields a data sheet that says the coil resistance is 25 ohms for a 3V and would use 120 mA at that voltage (25 Ohm * .120A = 3). Note if the same coil were run on 5V it would draw 200 mA (25 Ohm * .200A = 5V).
120 mA is bloody high to be coming from a logic pin which is why the MODULE has more than just the relay. What the MODULE likely contains is an additional gate device that takes less current that then supplies the relay (with 120mA) from another port.
You might also be looking at a solid state relay that uses less input current but generally has some slight leakage when "Off"
Do NOT try to drive a simple relay from an MMU like a ESP8266, Arduino, or Pi. That would likely destroy the digital pin on the MMU, and might destroy the whole processor.
You want a "digital relay module" which has a driver circuit to control the relay, plus some form of isolation to protect the digital pins.
A relay like this would be perfect: https://www.amazon.com/Icstation-3-3V-Optocoupler-Trigger-Development/dp/B01300ZY5M
Note that those come in 1, 2, 4, and 8 channel modules. Such a relay needs an external power supply, and your ESP8266 may or may not be able to provide enough current for the relay from its voltage regulator. Check the current requirements on the relay and the output specs on the ESP8266's voltage regulator.