changing my Honeywell Satchell tlx 2259 for a Honeywell DT90

[Grandad63]

There are currently three wires attached to my honeywell
Black to 1
Red to 3
Blue to 4
could somebody please tell me to which terminals I should attach them to on the Honeywell DT90E which has A B & C. Thanking you in anticipation

 

[holmslaw]

..

 

[Grandad63]

Yes positive about colours there is also a standard earth wire green & yellow attached tp the back of the unit
Were the three colours the old standard?
Red Live
Black Neutral
Blue Earth
as it is possible that this thermostat has been in the same place as the house was built approx. 1976
Where would I look to see where these wires originate from as of course they are plastered over in the lounge?
Thanks for your assistance anyway I will try it