7+3視覺英語網上英語課程，請問其中的 +3 （syntax）是講什麼？

Reference to the book referred by you TW Chen's the heart of grammar, you should make note that indep=co-or; indep=m.c.; dep=sub..
Herebelow are the identified website fuller definitions of  Ah Tao explaining 3 syntax terms:-eg:-
Compound sentence:-Main clause + 1 or more co-or(ind) clauses
Complex sentence:-Main Cl.+ 1 or more sub(dep) clauses
Compound-complex sentence:-m.c.+ 1 or more co-or clauses+1 or more sub cl.