Perhaps just try manual moving (use grouping to select all sheets at one go).
This may suffice w/o any complications, and can be done within 1 min.

Let's say sheets in book2.xls and book3.xls are to be moved to book1.xls.

Open all 3 books.

In book2.xls, hold down SHIFT key and select the leftmost, then the
rightmost sheet tabs. This groups all the sheets. Right-click > Move or Copy.
In the dialog, select book1.xls from the "To book" dropdown, position the
move as desired in the box below, click OK.

This completes moving sheets from book2.xls to book1.xls. Book2.xls will
close automatically. Any sheetnames in book2.xls moved over which are
identical to existing sheetnames in book1.xls will be auto-named with a
suffix by Excel, eg: Sheet1 (2).

Repeat steps to move sheets in book3.xls to book1.xls.