You got exactly what you ask for :-)
The doubled result in the result is because the recordset looks like this (Lets change the second 10 in the crsTest1 to 11):

Fld1     Test1.Ammount   Test2.Ammount
---------------------------------------
1            10                100
1            11                100

You got as many records as the largest number of records in all tables. In VFP9 you could use derived tables to got the result you want, I am not sure about VFP8 (can't test it right now).
Here VFP9 query:

SELECT crsTest.Fld1, cTest1.Amnt AS Ammount1, cTest2.Amnt AS Ammount2;
       FROM crsTest;
INNER JOIN (SELECT Fld1, SUM(Ammount) AS Amnt;
                   FROM crsTest1;
                   GROUP BY Fld1) cTest1;
ON crsTest.Fld1 = cTest1.Fld1;
INNER JOIN (SELECT Fld1, SUM(Ammount) AS Amnt;
                   FROM crsTest2;
                   GROUP BY Fld1) cTest2;
ON crsTest.Fld1 = cTest2.Fld1

If this didn't works in VFP8, you could first query second two tables and then join the results to main one.