I've seen the word for scissors "Hasami" written in hiragana " はさみ" katakana "ハサミ" and kanji "鋏". which is the most common?
はさみ and ハサミ are both very common, and you can use whichever you prefer. はさみ is definitely not a loanword, but there are words that are conventionally written also in katakana. Animal/plant names are the best-known example (see this and this), but there are other similar words (カンナ, ノコギリ, メガネ, クルマ, ...). Experts and enthusiasts may tend to use the katakana version more often, but it's hard to generalize. When in doubt, you can use a corpus like BCCWJ.
鋏 is not a joyo-kanji, and it's no longer common in modern Japanese. Some native speakers may not be able to read it.
I find a very useful tool for this sort of question is kanshudo.com. If you look up a word there, and then go to its "Details" page, it will list all the different ways the word can be written and give a bunch of details about which forms appear to be used most frequently, based on various corpus searches.
So if we look at the kanshudo page for はさみ, it tells us that (based on various searches), it is written as はさみ about 35% of the time, ハサミ about 30%, and for 鋏, it says "this form is rarely used".
Interestingly, kanshudo also lists another form which you didn't mention, 剪刀. It claims this form is also used around 30%, but looking at the search result data it lists it actually shows it has about the same number of Google hits as the "rarely used" 鋏, so I'm not sure where they're drawing that conclusion from. (My guess is that this form might be used heavily only in some specific contexts or something). This does show one thing you should be a bit careful of with this site, though. You do sometimes want to look at the actual numbers listed, not just the summary percentage.
So in conclusion, based on kanshudo's (automatically generated) data, はさみ, and ハサミ are both used about equally, but 鋏 is not really used much (and in some contexts, 剪刀 may be common too, but it's a bit unclear from just that site what those contexts are).