I went on a world tour and came back home

After you enter the right password, once again the door opens before you. You are not surprised to see another door blocking your path; looks like they want a three letter password this time. However, the part beneath the input sports a poster, with the following written on it:

                      "I went on a world tour and came back home"

TOEVAQCNXFLEFJRCUYAYQBZAOGAZATVFYHXEYVR???GUNNST


Can you outsmart the door once again?

• How much googling do you expect for this..? I'm pretty sure I've cracked part of it, but I am stuck trying to find a pattern :( – user812786 Aug 5 '16 at 17:32
• You could try to post what you did and maybe someone else can complete your reasoning and find the answer. – FrodCube Aug 5 '16 at 19:16
• @FrodCube I hopped in chat and found out Jonathan had gotten as far as I did, so I figured I should wait. And now it seems he has figured out the rest! – user812786 Aug 5 '16 at 19:45

UEI

Because
TOEVAQCNXFLEFJRCUYAYQBZAOGAZATVFYHXEYVR???GUNNST

rot13 and split into trigrams is:
GBR IND PAK SYR SWE PHL NLD OMN BTN MNG ISL UKR LIE ??? THA AFG

Which are all NATO country code trigrams:
United Kingdom, India, Pakistan, Syria, Sweden, Philippines, Netherlands, Oman, Bhutan, Mongolia, Iceland, Ukraine, Liechtenstein, ?, Thailand, Afghanistan

Their capitals are:
London, New Delhi, Islamabad, Damascus, Stockholm, Manila, Amsterdam, Muscat, Thimphu, Ulaanbaatar, Reykjavik, Kiev, Vaduz, ?, Bangkok, Kabul

The countries are linked by the next one's capital starting with the last letter of the previous one's capital (including "coming back home"), so the capital of the missing entry must start with "Z" and end in "b", well there is only one such capital...

Zagreb, Croatia

...and Croatia has the NATO trigram:
HRV

rot13 to get back to the text we were presented with (thanks hexomino!):
UEI

• Great answer. Are you sure it's not rot13 of that? – hexomino Aug 5 '16 at 23:15
• @hexomino - oops, you are probably right! Schoolboy error. – Jonathan Allan Aug 5 '16 at 23:24