With due regard to @Robusto's answer (which I don't totally disagree with), @palooka's comment thereon, and @Caleb's unassailable assertion that strictly speaking you can't use a single tense for past and ongoing actions simultaneously...
If you have to choose the best of these four, go for the third.
- 3 All three would agree that there is no deity.
The conditional "would" implies they'd only agree on this matter if they were asked. It's of no great consequence that in practice we can't retrospectively ask the dead ones. The whole statement is phrased as a conditional anyway, so it's all hypothetical, and needn't happen/have happened at all.
I'd be inclined to think this very sentence illustrates how casually we ignore the implications of the conditional tense in some constructions. The 'd there stands for "would", but I imagine many people hardly noticed it. Obviously I must have meant "am", not "would be". By the same token, we don't notice the conditional in OP's example - we just assume some kind of "present" tense.