Once you get some sort of sensor that gives you some sort of pulse signal, you have to look at whether that pulse signal can connect directly to a U3 digital input, or whether it needs some signal conditioning. The input low logic level of the U3 is -0.3V to 0.8V and the input high logic level is 2.0V to 5.8V:
If your voltage levels are bigger, you might need a voltage divider. If smaller, you might need a comparator. You also need to consider whether the signals are push-pull, open-collector (NPN), or something else:
Once you have a signal with valid logic levels, see the following forum topic about frequency measurement:
With the U3, I suggest you try a timer in the 32-bit Period mode. Start by trying it in the test panel in LJControlPanel.
If your load cell is a typical raw load cell, that means it is simply a bridge circuit. The U3 will need an LJTick-InAmp to get readings from a bridge circuit: