There are differences. First the easy one, ubiquitous. ubiquitous - occurring everywhere, omnipresent. It is not a word that is commonly (sorry, I could not resist using commonly) used in everyday conversation. Its use tends to be for something that is everywhere, all the time. "Insects are ubiquitous." "Air is ubiquitous." "Failure is ubiquitous." While I guess you could say that "Toyota cars are ubiquitous.", the typical way to state this is "Toyota cars are common." We tend to reserve this word for things that are not typically influenced by human intervention (i.e. Toyota could go out of business or restrict the sale of its cars, that will not happen to ants or air). common - belonging equally or shared amongst a group, community, nation, etc; joint, united; widespread, general or ordinary; a frequent occurence. "We share common experiences." "The USA and China have common interests." "Apple iPhones are common worldwide." "We have a common interest in the arts." Of the three words, common is, by far, used the most in all forms of communication. commonplace - ordinary and uninteresting; trite. Commonplace can have a negative tone to it. It is sometimes a sophisticated putdown. "That is common knowledge." This means everyone knows this. "That is a commonplace idea." This probably means, "Look, you idiot, everyone knows that and when you can bring new or interesting information to the conversation, share it otherwise please be quiet.". (OKAY, that was a little strong.)

I think commonplace means something that is very ordinary, of no great merit, easily to be found in many situations, even dull and uninteresting Uniquitoius is much more about something that can be found everywhere, almost omnipresent .

Getting rich is becoming a ubiquitous part of our national culture.