It's not clear whether OP looked at 3G signal strength in auto LTE mode or 3G only mode. But from my experience, it doesn't seem to matter much. Even in forced 3G mode, dbm level is close to that of LTE mode. Forced 3G mode seems to be a little better only in weak signal areas.
I guess signal strengths of 3G and 4G are not that different but 1X is noticeably stronger. Right now only LG does separate 1X, 4G signal strength reporting. I noted that 1X is about 15~20dbm stronger than 4G in Spectrum, which is about the dbm difference we see on Nexus and other 4G phones. so that seems to make sense to me.