You did not specify the filemaker version, if your using FM10 or FM11 you can use an OnObjectEnter script trigger for the field you want locked.
Create a script such as Block Fields A B
If [ not isempty( yourtable::C )
Go to Field[ ]
Exit Script 
Set the onObjectEnter trigger for Fields A and B to run this script. If you then tab in, or click into the field it will simply leave the field.
For the text, perhaps you could simply type 'wait' on the layout, format it as required, then add conditional formatting such as
Formula : isempty(yourtable::C)
Then apply a text size of 500px
This will cause the text to resize beyond the bounds you specify (thus becoming invisible) if field C is empty.
If field C has a value, it will show the text