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Q: 3000 giandel connected to universal 12v 200 ah battery. bought 200 a fuse...ok?

No, a 300 amp fuse is recommended for the 3000 watt inverter

Q: If I use 3 batteries, (100 amp hours per), will a 200 amp fuse between the batteries and inverter work fine?

Probably too late to help the OP but for the benefit of others I'll answer. As answered by another already; it depends. To go a little farther than that though, it is good to understand the philosophy of fuses or overload breakers. The concept is very simple: the fuse needs to sufficient to protect the weakest part(s) of the circuit. So if the weakest part of the circuit is an interconnect cable that can only safely carry 45 amps, then you need a fuse which will blow before allowing 45 amps to pass through, or at least for too long a time. In a situation like this involving batteries, it can also be important to know the limits of safe discharge rate for the batteries. You want to stay within the limits.

Q: pulling 120amp theough this it gets too hot to touch. any ideas why? I have two of them and they both do the same thing

Right up front I want to point out that my answer is based in part on speculation, so do not rely on it in any way that could affect a safety concern etc. That being said, my thoughts on it are as follows. The way fuses like this work is by melting apart a metallic bridge thus opening the circuit. The higher the current through any conductor the higher its temperature will be. The idea being the fuse is designed to allow current up to a certain point and then become hot enough to melt after that point. As Spock might say, logic dictates that the temperature gradually increases with current therefore a fuse passing no current would be ambient temp. and one passing near rated current would be close to its melting point. This means that a fuse passing current which falls between those would have a temp. between those as well. You cook food in a pan which is too hot to touch for sure, but well within its own normal operational limits. So the question is, is your fuse running too hot for its operational conditions at 120A? I don't know, but I would not use touch-ability as any kind of reference. The best answer is to ask the manufacturer what current & temperature relationships are considered normal for the part. The 2nd best answer would be to have an engineer with generalized experience with these types of parts weigh in. And then the last answer I would consider would be mine, which is measure the temp. at your typical current loads and just make sure it is less than needed to start combustion on any kind of materials which have potential to come in contact with it.