With 3 flexes you are going to want to load up on wide receivers and running backs. Depending on the value present, I could see opening up the draft RB, RB, RB in standard scoring. Is this a PPR league? If so then I wouldn't go so running back heavy in the early rounds but 3 RBs and 3 WRs would be a decent starting corps. If this league is non-PPR, I'd push hard for a total of 4 running backs in your roster. You may not to be able to get 4 feature backs, but I could see 3 starting RBs and then a TD vulture like Brandon Jacobs, Willis McGahee, or Mike Tolbert, etc.
I can't necessarily say which position between QB or TE will be "devalued" exactly. A lot of that depends on how the draft is going. You need to focus on WRs and RBs, so whatever value presents itself outside of that will shape your draft plan. If a QB happens to fall in the 4th to 7th round range where he has great value, put the trigger. The same goes for TEs.