In the 3-3-3 plan, what is channel 3?

Repeaters always publish their transmit frequency 

If the repeater receive frequency is higher, it is a Plus offset

Offset sign = Freq receive - Freq transmit

So using fictitious numbers

100r - 95t = +5 offset

On your radio you enter the repeater transmit frequency which is your receive frequency.

If you use the same convention:

R - T = offset

Rearranging 

R - Offset  = T

95r - 5  =  90, NOT 100.  (To make it 100t the transceiver would have to use a Minus Offset, the opposite of the repeater.)

So….there are three possibilities to make it work:

1. The transceiver does not use the same definition of offset.

2. The transceiver automatically inverts the sign of the offset.

3. You are actually entering the repeaters receive frequency not the transmit frequency.

I’ve never found a description of what it actually is.

Of course on some transceivers you enter the receive and transmit frequencies separately. In that case you just need to know how repeaters work.

Repeater listings always list the repeater's transmit frequency which is what a person will listen to. The repeater listens to a plus or minus offset, typically uhf is 6 mhz and vhf is .5 mhz. This is what the user must match for the repeater to hear and repeat. The user either goes up (+) or down (-) with offset of their transmitting frequency to match the repeater listening  frequency.

Yes, radios like baofeng require each frequency to be entered. Radios like Kenwood or Yaesu will make the switch by just entering + or - in the menu setting provided the offset is set correctly which is sometimes adjustable but also can have a factory default.

On two meter a + will transmit .5 mhz higher than the listening frequency and a - will transmit .5 mhz lower than the listening frequency.

Most radios that display the frequency, the difference will be displayed when xmitting or receiving.

It's pretty standard. 1)There isn't any inverting the sign of the offset.

2)The user listening  frequency is the repeater transmit frequency

3)On radios like baofeng , both frequencies need to be entered. 

I have heard for years how difficult baofengs are to program. I myself haven't had a problem.