Move that output.println line out of draw and into processByte. Like this:
void processByte( int inByte) {
serialInArray[serialCount] = inByte;
serialCount++;
if (serialCount > 2 ) {
meas1 = serialInArray[0];
meas2 = serialInArray[1];
meas3 = serialInArray[2];
output.println(map(meas1, 0, 255, 0, 1023) + "," + map(meas2, 0, 255, 0, 1023) + "," + map(meas3, 0, 255, 0, 1023));
serialCount = 0;
}
}
Where you have it now, it gets called every time you read a byte and that means that only one of the measure variables has changed