Recently my employer did an audit to the payroll records and realized
that the FICA withholding was less than required among other issues.
They want to deduct the not withheld amount for the last 3 years, from
our paycheck all at once. We will receive no paycheck for a complete
month. Can the employer legally solve the problem in this manner? We
did not fail to pay, the employer failed to deduct. Should there be a
payment schedule? We want to pay what we owe, but we cannot pay it all
at once and be late on our remaining obligations.

That seems completely unreasonable to me. They might deduct the extra
in 11 equal instalments, the last being a financing charge. I think
each employee should contact IRS to make sure your employer is not
pulling a scam on you. Your Repesentative and Senator may also be
helpful in reaching a reasonable compromise.   Neil