In the 1st and 3rd sentence, dem is the demonstrative pronoun das (or, much less likely but undistinguishable without more context: der) in dative case. It looks like it is referring to something that has been mentioned before. Canoo.net has a short explanation on how this works. The pronoun is declined into dative case because it is referring to something that plays the role of an object that has to be in dative simply because the verbs in these sentences (entgegenhalten and zustimmen, resp.) require it.
In the 2nd sentence, dem is the article der, again in dative case. The preposition aus always requires the dative case, so both "die hohe Anzahl von Spielern" and "der ungeheure Zeitaufwand" need to be set in dative case.