Can the verb be in third position?

Question

From a text I am reading:

Wer sich für Aktien interessiert, der behält den DAX genau im Auge.

Isn't this incorrect and shouldn't this be written

Wer sich für Aktien interessiert, behält der den DAX genau im Auge.?

Dartmouth Dept. of German states: "'wer' can be used only in the sense of 'he who' or 'whoever' - this is not technically a relative pronoun, since it has no antecedent; rather the whole clause in which it occurs occupies the position of a subject or an object." And then they provide examples in which the verb is in the second position, following the relative clause. This is the rule I understood to be standard. https://www.dartmouth.edu/~deutsch/Grammatik/WordOrder/relatives.html

Answer 1

You are right to say that a main clause following the subordinate clause will have the verb at position 1:

Wenn noch Zeit ist, gehen wir spazieren.

Wer Deutscher ist, weiß das.

In some cases however, you can begin postpositioned main clauses by a sort of correlate responding to the w-word of the subordinate clause and referring back to its contents:

Wenn noch Zeit ist, dann gehen wir spazieren.

Wer Deutscher ist, der weiß das.

Similarly, you can say

Ob ich wirklich komme, (das) weiß ich heute noch nicht.

Wann ich Feierabend mache, (das) ist allein meine Entscheidung.

This type of correlate has to be at/on (?) position 1 of the following main clause, otherwise you would have, as Kilian said, a question (your second example) or an impossible structure of two subordinate clauses (your third example).

Other examples:

Wo heute ein Supermarkt steht, (da) war letztes Jahr noch eine Wiese.

Wem das Buch gehört, dem musst du es auch zurückgeben.

And with a leading main clause:

Das ging alles so schnell, dass ich die Hälfte vergessen habe.

Die Epidemie ist so gekommen, wie die Experten es vorausgesagt hatten.

There are many other structures containing correlates, e.g. two-part connectors like zwar - aber or einerseits - andererseits. They connect two main clauses, though.

Answer 2

In my opinion the sentence is fine as it is. A link to the full text may help to understand your concerns. The first alternative is wrong, while the 2nd sounds outdated.

Answer 3

The sentence is fine as it stands. The part before the comma is a relative clause, and the part behind the comma is the main clause, and the main clause has plain vanilla SVO order.

Your second example has VSO order, which would be fine in a question, but not in a main clause.

Your third example has SOV order, which is is fine in a subclause, but again, this is a main clause and has to follow main clause rules.

Perhaps things are complicated by the fact that the construction you show can also use the relative clause as a subject:

Wer sich für Aktien interessiert, behält den DAX genau im Auge.

This is also fine, since the entire relative clause counts as the S in SVO, but admittedly this is confusing for learners of German.