My recommendation would be to wire the two front fans to a standard 4-pin molex female connector. Drawing that amount of current from a 3-pin motherboard header could exceed the rating of the fan header and damage the board.
Female Molex connectors are available from Maplin click here ref JW65V for £ 0.79 each. They contain the crimp solder terminals so you need to have or borrow a soldering iron.
The top fan can connect directly to an existing PSU connector.
An alternative would be to install a front panel with variable speed fan controllers and wire the fans to this.