How to check whether my arduino-mega ADC is sampling correctly?
In this case the baud rate will be real bottleneck. You have to send from 110 to 230 bits per reading (1 start bit, 8bits data, 1 stop bit)*(3*2 spaces + 4-16 number characters + 1 line feed).

That means best case (all numbers are less than 10) 349 readings per second. And worst case is 166 readings per second (all readings are more than 999).

So it's about 10% probability to be sampled correctly.

You can check if the next time (with misleading name lastReading) is less than current time. That means you are already too slow.