Vanilla 5e Pact of the Tome lets you choose any three cantrips from any three spell lists. That is, Tome Warlock can have any combination of three cantrips existing in 5e. In BG3, These cantrips are pre-set to Thorn Whip, Vicious Mockery, and Guidance. You're just not gonna use the first two over Eldritch Blast
Vanilla 5e Pact of the Chain gives you the additional familiar options of a Sprite and a Pseudodragon, this is important because these are good- and neutral- aligned creatures. In BG3 the only pact-specific familiars are Imp and Quasit, which are Lawful Evil and Chaotic Evil familiars. Like why would your patron be an Archfey and you not have a Sprite? Sucks for roleplay T B H