transmission slips in 3rd,
Time to take it into a someone who specializes in transmissions in your area. Sounds like it's time for a rebuild.
First, check fluid level. If OK, then it's shop time. Could be something as simple as a band adjustment, new filter and pan gasket. Could be a sensor, electronic controlled transmission. And it could be the trans is toast and needs a complete rebuild. You didn't say how many miles were on it, and how hard those miles were...
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