Look in the Electrical Specifications section of the datasheet for the PIC you're using.

If the USART RX pin is an ST Schmitt Trigger type, it's going to require signal levels
somewhere from ground to 0.2 * Vdd for a logic low, and 0.8 * Vdd to Vdd for a logic
1 level input.

If your external device operates at 3V, and can't provide these logic level signals, you
may need some kind of level shifting like a small signal transistor or FET between the
two. Or just select a PIC that can operate from a 3V supply.

Here's a nice Microchip app note with tips & tricks for 3.3V interfacing;
http://www.microchip.com/stellent/gr...c/en026368.pdf

Look at Figure 6-1 for a very simple 3.3V to 5V MOSFET translator. For non-inverting, just
move the resistor/PIC input to the source side VS drain side.