I have a Raspberry Pi 3 and a display that has only RCA as inputs. Please help me connecting the Raspberry Pi to the display so that I get both sound and visual.
a display that has only RCA as output
I presume you mean 3 female inputs, 2 for audio and one for composite video, which in North America are generally red, white, and yellow.
The 3.5mm jack on the Pi 3 can be connected to this as it includes a composite video line. Jacks like that can be used with a normal 2 channel audio only cable, or a 3 channel one such as that shown below (the two ends are from the same cable).
Notice the three bands on the 3.5mm jack, one more than a standard stereo plug (these are actually separators as I believe in both cases there is an additional pole for ground, which is the sheath on the RCA end). These are commonly available at electronic stores or online (search for a "3.5mm to RCA video cable").
Beware as mentioned here (thanks Milliways) some of these cables are intended for camcorders and have one of the rings on the 3.5mm jack swapped; those will not work. So don't get a cable that is intended for use with camcorders!
To be sure, you can get a cable from Adafruit to ensure that the cable is for RCA display product (versus RCA output product).
I am running a raspi B+ running retropi for a retro game station in my mancave and I have a dinosaur TV that only has RCA. I ordered a cable from Adafruit and it works like a charm. Cheapest and best one out there: https://www.adafruit.com/product/2881
All you do is plug it into the audio jack on the raspi and then into your RCA and you're set!
It's a great alternative to HDMI to RCA converters. Much less hassle.