Your Fit 3 only shows a preview because the full display is blocked in the Samsung Health app or the smartphone settings for notifications (e.g. "Do not disturb", energy saving mode, background data restrictions), not directly in the watch itself. "Do not disturb", power saving mode, background data restrictions) is blocked, not directly in the watch itself; so you need to check and adjust the notification permissions for WhatsApp and the app settings on your paired smartphone to see messages in full.
Troubleshooting steps (smartphone settings):
Check WhatsApp notifications: Open WhatsApp, go to Settings > Notifications and enable pop-up notifications (for messages and groups).
Phone settings for notifications: Go to your smartphone settings > Notifications > App notifications and make sure WhatsApp is enabled here to show banners and sounds.
Background activity & power saving mode: Power saving mode: disable it as it restricts background activity, including WhatsApp. Background data: Make sure that WhatsApp is allowed to use background data (often under Settings > Apps > WhatsApp > Mobile data or Battery).
Samsung Health App: Open the Galaxy Wearable app (or Samsung Health), go to Device Settings > Notifications > App Notifications and select WhatsApp to make sure it's enabled for your watch.
Testing: Get a message sent to you. Sometimes restarting the phone or watch after the changes helps.
If you adjust these settings on your mobile, the Fit 3 should be able to display the messages in full, as the watch only mirrors the phone's notifications.