International Checkers - winning strategy for 3 [4] kings vs 1 king
The answer by @RemcoGerlich is essentially correct. Some extra info below. For further reference see the Dutch book Drie tegen een is gemeen, that contains a mathematical proof that 3 vs. 1 kings is a draw (which predates the age of perfect knowledge endgame databases by almost a decade!).

The answer depends crucially on both the board geometry and the king movement.

For short-ranged kings (American checkers and Italian draughts) 2 kings can force a win against 1 king. The same applies for orthogonally capturing kings (Frisian draughts) and kings that need to end a jump immediately behind the last jumped piece (Thai draughts).

For long-ranged kings, on square boards larger than 8x8 (10x10 International and 12x12 Canadian draughts), 3 kings against 1 king is a draw but 4 kings can always force a win.

On an 8x8 board (e.g. American pool checkers and Russian draughts), it is a win if the majority side occupies the long diagonal, and a draw otherwise. The key position is called Petrov's triangle (in Russian, use a browser translator):



On rectangular boards of dimensions N x (N ± 1), N x (N ± 2) and N x (2 N - 1), there are two single corner diagonals that allow special tactics to enable a 3 vs. 1 forced win.

In International draughts, the 3 vs 1 drawn engame is seen as the main culprit for the &gt;90% drawing percentage at grandmaster level matchplay. Many game tweaks have been proposed.