They'll give you a bullshit answer for all of those. For your first argument, they'll tell you it has to do with the 'law of perspective.' I think it's safe to say we've debunked this one, unless they come up with some good counter-arguments soon, but that's very doubtful. See "waves, crests and sunsets - unanswered" and "debunking the 'law of perspective'". This pretty much leaves them without an explanation for why the sun sets, why ships dissapear beyond the horizon etc. For your second argument they'll tell you that the sun's rays aren't constant and that the sun circles the earth only a few thousand miles above. For the third argument, they'll say that just because all those others are round, it doesn't absolutely prove that the earth is as well.
Don't hold your breath for them doing a better job. Once you give them some thought, these theories hold no water.
Also keep in mind that there are probably fewer members who actually believe in a flat earth than you think. Some of them are without a doubt just trying to be funny and their posts are just satire. With others it's hard to tell. But in any case, even some members who seem to argue for a flat earth, aren't really flat earthers.