In 2014, I published a how-to article on this same subject. The article was based on my experience with the Motorola Moto G. See How to open the Motorola Moto G to 3G-only. While most Android smart phones let you peg network mode to 3G only (also referred to as WCDMA only), some manufacturers leave out that option. Motorola is one of them.
On both the Moto G and Nexus 6, the option available is to set a preferred network mode, meaning that the phone searches for that mode first and falls back on any lower one failing to find that. Pegging a phone to a network mode means that it stays in that mode all the time.
That 2014 article pointed to an app called Notification Toggle that solved the problem for me. However, that app has since been updated and does not have that feature any longer.Or at least, I couldn’t find it (if it still is there, do holla). Well, I found another app: Radio Switcher.
Should your Android phone not let you peg to 3G only, with Radio Switcher and other similar apps, you can do so easily.