If our eyes can see the signals, we will see 3 INDIVIDUAL stream of signals coming out from the router representing each frequencies. it's just that our devices by some standards (
IEEE 802.11
blah blah)
these 3 signals will be listed as one, that's why you see only 1 SSID in your wifi list...
in older devices that can only do 2.4 ghz, then they can only see the 2.4 ghz signal.
so to answer your question, your older smart device will see only the 2.4 ghz and will connect to it without any problem.