So just created a new flower bed and will be transplanting my 3 yr tree peony. I live in the PNW. Any advice would be appreciated.

Look up on-line, ground prep. for a Tree Peony.
IF, if it grows well , they can get BIG..

Preparing the soil deep and well is most important.

Looks like a great spot. Similar to where I have planted mine net to a shed to give them shade in hot sunny summers. They grow in any medium. Mine are planted in clay soil with zero amendments add for 20 heats. They only issues I have are the around squirrels and mice. The deer don't bother them which is a plus in the PNW.

I've never grown tree peonies... so I'll throw that fact out right now!

But-
How close to winter are you?
How harsh are your winters?
I'd think in my area, if I had the choice, I'm both too late and too early to transplant.

Let me explain:
In my zone 7b area, it's that 'tween' time for perennials. In between hot summer and cold winter. They are adjusting to the cool (cold?) fall by preparing themselves to go dormant, or at least to go to a resting period.
I begin to stop watering, and haven't fertilized for months to give them the cues that winter is coming. Fresh new fall growth right at the start of winter doesn't usually work out well.

Don't get me wrong- fall is generally a great time for transplanting. I like to get them transplanted EITHER by the last of August, so my perennials have a chance to adjust and set out some new root growth, or wait until they go 'soft dormant,' having experienced frosts and lost their leaves. In this case, when they wake in the spring, it's time for growth in a new home.

Maybe your area is warmer and this isn't a concern for you!
I hope others will weigh-in.

In Minnesota, we plant all peonies in the fall, October is best.
I cover them with straw for the first two-three years but here we get -15 below and worse most winters.

If you have to transplant a tree peony, now is the time. Get a really big root ball. They hate to be disturbed so that they recover.