Battery supplying different voltage
You don't show where the grounds are, but if it's a common ground, then no diode would be required, just tap off two batteries for about 3V and three batteries for 4.5V. 

It may be not be a desirable thing to do because the 3V batteries will be exhausted before the "extra" battery and this might cause strange behavior in your circuit unless you separately detect voltage brownout conditions on both supply lines. If you don't care about the user having to replace the 3rd battery unnecessarily, it may be okay, but you should also parallel (not series) the 3V pair with a reverse-biased diode so that the 3V bus can't go negative if the 3V batteries are exhausted and a heavy load is applied to the 4.5V bus. 

The diode type would depend on the maximum current (including fault current). If you have a polyfuse in there, a 1N5819 may be fine. If the batteries can be inserted backwards, the diode will conduct large current and may get very hot and fail unless you have something like a polyfuse in series.