Oracle 8.1.7.0.0 - INSERT using SELECT

I'm VERY new to Oracle. I've been a SQL Server programmer for quite some time but don't understand the intricasies of Oracle.

Here's my problem:
I've created a temporary table TMP_MYTMPTABLE with the following field definitions:
TAGNAME VARCHAR(32),
TAGDATE DATE,
TAGVAL NUMBER

Now, I want to insert records into this table using a select clause building the fields by concatenating values from joined tables.

Here's and example:
insert /*+APPEND*/ into TMP_MYTMPTABLE
(TAGNAME,TAGDATE,TAGVAL)
VALUES
(
SELECT SUBSTR(MYTABLE1.FIELD1,3,2) || 'L.' || Lpad(TABLE1.FIELD2,3,'0') || '.' || TRANSLATE(TABLE2.FIELD2,'-','_') || '.' || TRANSLATE(TABLE3.FIELD1,'-','_'),
to_char(MYTABLE1.FIELD4,'MM/DD/YYYY') || MYTABLE1.FIELD5),
to_number(decode(TABLE3.FIELD6,'L0.01','0.0', TABLE3.FIELD6))
FROM MYTABLE1,TABLE2, TABLE3
WHERE MYTABLE1.FIELD1 = TABLE2.FIELD1
AND TABLE2.FIELD2 = TABLE3.FIELD2
AND TABLE2.FIELD1 IN ('TEST')
);

When I run this, I get the following:

ORA-00936: missing expression

If I just run the select statement, it works fine! Could Oracle be getting confused with all of the functions and concatenations?
Thanks