Ok, here is how I have done it.
 
1. Plug flap servos into channel 7
2. In the settings menu go to Aux-Ch and set channel 6 to throttle hold, this will be "flap" channel for the tx programming BUT not for the plane  - that will be ch 7 as above.
3. Now set ch 7 to pitch trim - this will be aux 1 on tx
4. By doing this you will have a flap over riding switch - throttle hold to give full flap up/down if you need it.  You also get the pitch trim pot for minor adjustment of flap position. (set it to the mid position)
5. In program Mix do the following
 mix 1
set act
master flap
slave pitch trim (aux 1)
dn rate +036
sw nor
 
mix 2
set act
master flap
slave pitch trim  (aux 1)
dn rate -24
sw idl1
 
mix 3
set act
master flap
slave pitch trim  (aux 1)
dn rate -100
sw idl2
 
Operation - the 3 position switch will now operate the flap servo to the 3 positions programmed.  Keep throttle hold switch off to use 3 position switch.  The flap servo position can be programmed using the dnrate setting.  The above will get you going and you can fine tune the settings.
 
Remeber that the pitch trim pot is always active and if you bump it it will reposition the flaps.  Use it to get say zero flaps fine tuned and note the position and leave it alone.
 
You can go into the display menu and operate the switches and you will see that the ch7=flaps moving to the three positions and see the affect of the pitch trim knob as well as the throttle hold switch.
 
What this program mix is doing is taking the throttle hold off position as master and as you operate the three position switch it is causing a proportion of that to be slaved/fed to the pitch trim which you have programmed as channel 7 connected to the flap servos.

First up you'll need an 8ch receiver for this to work and

I'm using the stock firmware on my Turnigy 9x v2 actually, that sadly has a broken Butterfly feature.

However no need to throw it away just yet, as building program mixes yourself in Motor plane mode
(not Glider mode of the transmitter, so you'll have to change that in SYSTEM for your model) works very well and
as additional bonus you'll get to use the Throttle Hold switch for added safety for when your motor/throttle is concerned as well.

For usage of the flaps (one position for in-flight / soaring) and butterfly (full on brake for landing or brake whilst diving) we're going to use the three point switch. Which has three modes you can use, Normal mode, ID1 mode and ID2 mode.

Basically the following mix uses up all 7 Prog.Mixes in Motor plane mode of the T9x v2 and uses up 6 channels. In my case needed the Turnigy 9x v2 8 ch receiver for it to work.

ch.1 aileron 1,
ch.2 elevator,
ch.3 throttle,
ch.4 rudder,
ch.5 empty gear channel reserved for transmitter programming,
ch.6 Flaps with Y-cable,
ch.8 second aileron

(The 7th channel for some reason only provided ~70% of servo range, and using the same programming on a 6ch receiver I haven't as of yet got it working because of the reserved 'dummy' channel used in the programming)

The only minor issue with this setup is that it's impossible to mix elevator into these mixes unless you want butterfly as the only option, as you've only got 7 prog. mixes at your disposal. But frankly, this plane doesn't really need elevator being mixed into flaps and butterfly that much anyway.

It's easy getting used to just by compensating elevator yourself.

I'm flying Mode 2 with throttle on the left stick by the way.

The Prog. Mixes are as follows;

Prog. Mix 1:

State: ACT
Master: AIL (ch1)
Slave: AUX (ch8)
Offset: 000
Uprate: 100
Downrate: 100
SW: on (means 'always on', used for the Aileron mixing)

Prog. Mix 2:

State: ACT
Master: GYR (dummy channel)
Slave: FLP
Offset: 000
Uprate: 100
Downrate: -100
SW: On

Prog. Mix 3:

State: ACT
Master: GYR
Slave: FLP
Offset: 000
Uprate: -100
Downrate: 100
SW: ID1 (middle location of 3 pos. switch activates Flaps now)

Prog. Mix 4:

State: ACT
Master: GYR
Slave: FLP
Offset: 000
Uprate: -100
Downrate: 100
SW: ID2 (down position activates butterfly of flaps and ailerons, ..)

Prog. Mix 5:

State: ACT
Master: GYR
Slave: FLP
Offset: 000
Uprate: -100
Downrate: 100
SW: ID2 (.. and you need this mix twice for maximum servo movement for the butterfly)

Prog. Mix 6:

State: ACT
Master: GYR
Slave: Aux (ch8, the other aileron for Butterfly upwards mixing)
SW: ID2
Curve: Tune (press + to enter / edit this;

Points are; L: 0% , 1: 50% 2: 50%, 3: 50% H: 0%

(press Menu to save and go back to Prog. Mix 6, then press Menu short again (!) to save Prog. Mix 6)

Prog. Mix 7:

State: ACT
Master: GYR
Slave: AIL (ch1 aileron upwards mix for butterfly)
SW: ID2
Curve: Tune (press + to enter / edit this;

Points are; L: 100% , 1: 50%, 2: 50%, 3: 50%, H: 100%




(Again press Menu shortly to save the curve and then press Menu AGAIN shortly to save Prog. Mix 7.

You will also have to assign channel 5, the dummy channel, to a switch on the transmitter like for example Throttle Hold in the AUX / CH menu.

This makes sure the dummy channel is active, even if the 'SW' set to 'ON' should already do that.

Afterwards, do yourself a favor and activate the Throttle Cut / Throttle Hold feature of this transmitter as well in it's own menu and you're totally good to go.