I am by no means an authority on World Coinage, (heck, I have no idea what the second coin is), but FWIW, the 3 Gulden has the look of the Chinese fake Morgans (et al).
Wow. Some nice coins, danoomistmatiste. Decent photos too. Try to edit any future photos thusly - The first one - the 1831 three gulden - looks good to me. If in your shoes, I'd want to weigh it (it should weigh about 32.298 grams). The diameter should be 40 millimeters. There should be a dash between the crown and the shield, which I cannot see on your photo but that doesn't mean it is not there. Check out https://en.numista.com/catalogue/pieces6552.html - the coin pictured there has the dash. I will remain silent on the second one since I cannot read Arabic and the coin is not familiar to me. All I can make out is the AH date - 1299, which is November 1881 to November 1882. I think. Welcome to Cointalk! David
Yeah, I think I may be seeing what Beefer518 is seeing. Willem's hair just looks off for some reason; I cannot specify what it is. And the three in the 1831 doesn't look right. The weight and diameter are important. Is it magnetic? Again, I cannot offer you anything about the coin with the Arabic writing.
First coin looks very wrong to me - the shape of the jaw is way off the genuine examples and looks "cartoon-like". (Someone with a computer trying to make a copy without much artistic ability.) Second is more difficult to see what is wrong - the denticles in the rim look a bit deep and irregular? If it came from the same source as the first I would discount both. Interestingly the Numista page for the 3 Gulden includes an Ebay listing for a similar coin that is also clearly a fake.
The Zanzibar Riyal is a $1,000 coin. The one you have pictured appears to be a questionable specimen available from at least one ebay seller from the The Russian Federation for $75.00. I have doubts about the authenticity.
Oh, my. Here's the lowdown on the Zanzibar - https://en.numista.com/catalogue/pieces18071.html. It should weigh 27.16 grams. So, danoomistmatiste, if I may ask, how, where and from whom did you purchase these two coins from? How much? No specifics necessary. Ballpark figures. Do you have access to a scale that measures to at least the hundredth gram? That, and whether or not magnetic, would provide quick diagnostics. If your really curious, here are directions how to determine, within a reasonable degree, the metal content of those two. https://en.numista.com/numisdoc/determining-the-metal-of-a-coin-27.html - but again, you'll need a good scale.
The 3 Gulden looks suspicious. Tooled,harshly cleaned,or copy. I would weigh it and mic it... then get back with us.