Parallel:
Speaker 1 plus -> Amp +
Speaker 2 plus -> Amp +
Speaker 1 minus -> Amp -
Speaker 2 minus -> Amp -
Total impedance w/ two 4-Ohm speakers: 2 Ohms
Serial:
Amp + -> Speaker 1 +
Speaker 1 - -> Speaker 2 +
Speaker 2 - -> Amp -
total Impedance w/ two 4-Ohm speakers: 8 Ohms
Now, if you BRIDGE the Amp, each of the two bridged channels "sees" only half the impedance of the above, i.e. 1 ohm/4 ohms respectively.
Wiring two 3-ohm speakers in parallel will produce 1.5 ohms in normal mode, or .75 ohms per channel in bridged mode.
Many Amps are only 2-Ohm stable, i.e. they will not handle the current that comes with a lower impedance setup for a very long time. The higher current produces more heat and the amp can not dissipate the heat quickly enough.
It is normally not a problem if the overheat protection shuts the amp off multiple times (my sub amp does it quite regularly when I run it at full power for an extended period of time). However, if the amp heats up too quickly you can still fry some transistors before the protection circuit reacts.
HTH
Joerg
