Using CASE With INSERT,UPDATE Statement

Is it possible to use CASE statement with INSERT /UPDATE statement?this is what i am trying to do

i have a table like this

Field1 Field2 Field3  FieldType1 FieldType2 FieldType3

1-when there is no data for a given combination of  Field2 and  Field3,i need to do "insert".i.e the very first time and only time

2-Second time,when there is already a row created for a given combination of Field2 and  Field3,i would do the update onwards from there on.

At a time value can be present for only one of the FieldType1,FieldType2,FieldType3) for insert or update

I am passing a parameter to the stored procedure,which needs to be evaluated and wud determine,which field out of (FieldType1,FieldType2,FieldType3)has a value for insert or update .this is what i am trying to do in a stored procedure

CREATE PROCEDURE dbo.StoredProcedure

 (
 @intField1 int,
 @intField2 int,
 @intField3 int,
 
 @intFieldValue int ,
 @evalFieldName varchar(4)
 
 )
So i am trying something like

Case
WHEN @evalFieldName ="Fld1" THEN
INSERT INTO TABLE1 (Field2,Field3,fieldType1,FieldType2,FieldType3)values (
@intField1,@intField2,@intField3,@intFieldValue,cast(null as int) fld2 ,cast(null as int) fld3)

Case
WHEN @evalFieldName ="Fld2" THEN
INSERT INTO TABLE1 (Field2,Field3,fieldType1,FieldType2,FieldType3)values (
@intField1,@intField2,@intField3,cast(null as int) fld1 ,@intFieldValue,cast(null as int) fld3)

Case
WHEN @evalFieldName ="Fld3" THEN
INSERT INTO TABLE1 (Field2,Field3,fieldType1,FieldType2,FieldType3)values (
@intField1,@intField2,@intField3,cast(null as int) fld1 ,cast(null as int) fld2,@intFieldValue)

END

similar trend needs to be followed for UPDATE as well..
obiviousely its not working,gives me synatax error at case,when,then everywher.so can someone suggest me the alternative way?..i am trying to avoid writing  stored procedure to insert/update for each individual fields..thanks a lot