How to find threshold voltage of an NMOS with BSIM3v3 model in LTSPICE
Ground the source, connect the drain to the gate, and feed the drain with a constant current source set to the current value at which you want to know the threshold voltage. 

For example: 

In Circuitlab this shows a threshold voltage of 4.01V which is actually on the high side of allowable spec. 







simulate this circuit &ndash; Schematic created using CircuitLab