First, generic questions or questions without many details waste our time trying to figure out what you want. If you want good answers, then you need to provide good details.
What I would do in the situation that you describe is to program a Terminal huntgroup with a pilot number of 8100. Then place exts 5000, 5001 and 5002 in this group in the order that you want answered.
Without knowing your environment, I would still suggest a different method that may be better. Create a terminal hunt group with a pilot number of 8100. But, instead of 5000-5002 as members of the hunt group, create three new "dummy" numbers. You create these dummies numbers by first programming a dummy number as a "single line" on one of the phones. Then make "KEYLINE" appearances of that dummy number on the other sets. Once this dummy number exists in the system, it can be added as a member to the 8100 hunt group. Then, when a call goes to 8100, all the phones will ring on the first dummy number, then the second dummy number and so on. This will allow one phone to be able to answer more than one call.
One more tip... the help files in the mitel 3300 are very good! Reading up before asking on this forum may save some bitting responses.
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If you did not take enough time to get it right the first time...
What makes you think that you have time to fix it?