Manufactured products tend to have two spikes in their failure rate: soon after being put into service (manufacturing defects showing up when the product has been used a bit) and near end of life (parts are wearing out due to long usage). In the middle, the failure rate is pretty flat.

Monitors failing within 3 months suggests that you're seeing the infant mortality part of the curve. If a particular monitor lasts 3 months, it will likely last for many years.