dbexecute insert (Documentation Bugs and Problems)
Your problem is that the BASIC256 variable 'n' is not understood by SQLite. You need to build your SQL statement using string concatenation. Try this:
dbexecute "create table dicktest1 (id integer, nummer text, value decimal);"
for n = 1 to 3
stmt$ = "insert into dicktest1 (id, nummer, value) values (" + n + ", '" + n + "', " + n + ");"
You could do it all in one statement but sometimes I like to see the SQL statement before it executes.