Can LGA1151 motherboards only use DDR4 RAM?

It depends on the motherboard you can buy motherboards that are lga1151 that support ddr3. i recently bought a motherboard for a 6700k (which is lga1151) and put ddr3 ram in. but if you have the money i recommend buying one that supports ddr4 anyway because its faster and only a little bit more expensive. another thing to look out for is the pin type of the ram eg. 240 pin or 288 pin. check your motherboard specs and it will tell you all the different types of ram it can support. btw. the difference between ddr3 and ddr4 is not that substantial it wiont affect performance too much so dd3 will be fine for a normal build it will really only make a difference if you are planning on doing ram intensive things like video rendering and stuff.

the mobo are slotted for ddr4 ram and if not mistaken not compatible for standard ddr3 ram but support for ddr3L ram the problem is the voltage being applied using other ram besides ddr4 might even damage the processor

DDR3 will not physically fit into a DDR4 slot. DDR4 memory has 288 pins and DDR3 has 240 pins. Most 1151 boards use only DDR4, and a select few use only DDR3, but a DDR4 board cannot use DDR3 memory and a DDR3 board cannot use DDR4 memory.

http://lmgtfy.com/?q=can+lga+1151+use+ddr3+ram

sorry, I just had to xD