Why did I lose power to half the house when the breakers did not trip?
First, do the easy-to-check steps:  does a device that should be on still work?  Is the "on" switch on?  Is it plugged in?  (Children are often noted for unexpectedly changing these.)  If the device is plugged in somewhere else, does it work?  Does a portable device known to work properly suddenly not work in the alleged "failed" outlet?

Next, are you sure the breakers have not tripped?  Many manufacturers build them so that they do not visibly change when tripped.  To be sure, turn the lever all the way off and then back on.

Three circuits!  There is very little in common such that only three circuits with three separate breakers are all affected.  Maybe there are more?  Are there any other circuits which are on the same bus in the breaker panel also not working?  That would indicate a bus being disconnected, probably by a loose wire inside the panel.  A visual inspection with the panel cover off might be helpful.

Also possible&mdash;maybe probable&mdash;is that some neutral wires have come loose in the panel.  Turn off the main breaker and give all the wires a wiggle, being very careful not to touch near the service wires or where they connect to the main breaker.

Maybe something changed recently?:  Electrical work, ground work, utility work.