In 2004 there were just short of 114,000 polling places in the United States. The number has doubtlessly grown considerably since then, but take 114,000 for a fact even though 12 years out of date and all it takes to get to 3 million is 26 illegal votes per precinct.
Fraud is complicated by in-person early voting in 21 states and DC, making it possible for determined fraudsters to vote multiple times in one state and/or states. Alternative voting methods other than by in-person on election day are now estimated to account for more than a third of all votes cast. These include absentee/by-mail voting in many states, which liberals typically find more susceptible of fraud than in-person systems.