I assume the 3-year multi-entry means that they can come for up to 6 months per visit with unlimited visits for those 3 years correct?
No!! It means nothing of the kind. “Unlimited visits” would be an egregious abuse of the B visa, and it would be revoked. They would likely be sent back on the next flight.
How can they afford to stay for 6 months, if they have jobs and responsibilities back home?
The length of stay will be granted at POE by CBP. If they did get a 6 month stay, the recommendation is to leave before the date on the I-94, and spend at least a year out before trying to come back.
Please do some reading about this. If your in-laws plan to use a visitor’s visa to try to live in the U.S., it will be rightly denied or revoked. Abusing the terms of tourist visas can lead to issues for them later if they want to come back. If they want to spend more time in the US than for normal vacations, they will need immigrant visas.