magsafe 3 macbook air wont stay in charge mode

Two reasons.

1. The decision to replace or repair a Mac is at Apple's sole discretion. Since you are in the US refer to Apple's US warranty.
2. There may be nothing wrong with it. Read If battery charging is paused or on hold on your Mac - Apple Support.

markfromtowanda Said:

"magsafe 3 macbook air wont stay in charge mode: Still under warranty, why wont they send a replacement"

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About Replacements:

A. Serviced Items:

If you are awaiting completion, then go here to track it: Apple Repair and Repair Status Check - Official Apple Support

B. Replacements:

If you are awaiting a replacement in the mail, then use the link they would have sent you for tracking it down by the carrier.