Warning Message

Hi,

You can use "Data Validation" for this.

1. Select cells P3:R3
2. Select Data -> Data Validation
3. Validation Criteria -> Allow Custom
4. Enter the Formula - >=COUNTA($P3:$R3)<=1
5. Enter the "Error Alert" you want to see on the "Error Alert" tab
6. Ok

once you complete the above steps, copy cells P3:R3 paste it down the whole range.

See the below image and attached file for more details.