Why Capacitors in series combine like resistors in parallel: 1/C_total = 1/C1 + 1/C2 + 1/C3. For N identical capacitors that reduces to C/N, so 100 microfarads divided by 3 gives about 33.3 microfarads. Series always yields a total smaller than the smallest single capacitor, because you are effectively increasing the plate separation.
Watch out The choice of 300 microfarads is what you would get by adding them, which is how capacitors behave in parallel, not series.
Caps are backwards from resistors: series divides, parallel adds. Equal caps in series: just divide by how many.