3 sensor codes

98 trans port has Egr, mass airflow sensor, and o2 sensor codes popping up. I've replaced all three and codes still active. First thought was the air filter, so I replaced that and codes still active. My next thought would be the catalytic converters might be clogged but unsure if that idea would set all these sensors off at the same time. Anyone knowledged on this? Help!?

Did all 3 sensors throw the codes suddenly, and at the same time? If so, having all 3 go south simultaneously is rather rare. However, wiring issues common to these sensors could cause the problem, and given the age of the vehicle, a very likely problem. Not familiar with your vehicle, but if you could locate a wiring diagram, and see where the power and ground circuits to these 3 systems are common, and trace the wiring. On the older U bodies, the bulkhead connector for engine to body is a frequent source of problems such as this. Sometimes a quick way to find it was to wiggle, push, pull, squeeze the wiring bundle as the engine is running, and see if engine performance changes or CEL comes on. Be careful, however, of rotating parts as you do that.
Jeff

Jeffry's post brings the attention to the fact that codes very often point to the consequence of some failure, sensors affected, rather than pointing to the cause....so pay due consideration to this...before paying for nothing.