PIC controllers that run on 5V generally have two types on input.
TTL level and Schmit Trigger inputs (TTL and ST)
You can find the type in the I/O port section of hte data sheet.
A high input on a TTL input should be at least 2 V. A high input on a Schmit Trigger should be at least 0.8 x supply, which is 4 V with a 5 V supply. So it would be best to connect the 3 V signals to TTL input ports, but as Nigel says, it might work anyhow on the Schmit Trigger inputs.
It would certainly work if you reduce the PIC supply voltage a bit.
You can see the specified figures and typical values in the "Electrical Specifications" and "DC and AC Characteristics Graphs and tables" sections of the data sheets.