Ouch! have been studying this myself lately trying to update the wifes sora to 105. Check out shimano.com. Some of what i've learned may apply to tiagra check it out first.

shifters - 105 is different for 2 and 3 chainring
front derailler - 105 is different for 2 and 3 chainring
rear derailler - 105 is different for 2 and 3 chainring
cranks\bb - different for 2 and 3 chainring

my suggestion would be to keep your 2 chainring and change the sprocket sizes to match your gearing needs then upgrade to ultegra deraillers front and rear if you want. that could all be done for about $150(rough estimate).