What makes a good B350 board has a lot to do with what CPU you're putting into it. Only a few make the grade for R7 chips. R5 chips . . . you have more leeway.
That Gigabyte board, for example, appears to have a low-quality 4+3 phase VRM more intended for Bristol Ridge than anything else:
On the Gigabyte AB350-Gaming 3, the main PWM controller is an Intersil ISL95712 which can be configured for up to 4+3 phases and it uses all of that on this motherboard. Four phases are used for the main CPU and three are for the CPU NB. Each CPUphase has a single OnSemi NTMFS4C10N and two NTMFS4C06N (two are located in the back), while the CPU NB/SoC has a single NTMF4C10N and NTMFS4C06N FETs. Additional Intersil ISL6625 drivers are used to augment the integrated drivers on the PWM controller. All of the input capacitors use 5K Japanese FPCAP solid while all the other capacitors including the output capacitors are Taiwanese Apaq capacitors. The audio subsystem is an exception as well, using Japanese Nippon Chemicon audio-grade capacitors. There is no heatsink installed on the CPU NB MOSFETs and there are no mounting holes to install one later on.
http://www.modders-inc.com/gigabyte-ab350-gaming-3-motherboard-review-fun-flexibility/2/
Yup, that's weaksauce power delivery right there. You can probably get away with an R5, but I would not put a 1700 or higher in there. Just sayin.
As for the other boards . . .
MSI B350 Tomahawk:
MSI uses a 4+2 phase power delivery system to feed the AM4 CPU and its accompanying components such as the SOC voltage rail and (probably) iGPU for upcoming APUs.
A Richtek RT8894A PWM controls the system. Four CPU phases use four NIKOS PK616BA and eight NIKOS PK632BA MOSFETs, in addition to a component marked ‘4P=3H V38’ that I could not find information for but is likely a MOSFET driver. The 2-phase SOC power is handled by four NIKOS PK616BA and four NIKOS PK632BA MOSFETs, in addition to a pair of the aforementioned ‘4P=3H V38’ components likely to be MOSFET drivers.
This is a decent enough power delivery solution for a roughly £100 AM4 motherboard. The primary MOSFETs are the same type used on MSI’s £300 flagship X370 XPower Gaming Titanium motherboard. That point can be interpreted as a good thing for the B350 Tomahawk or a bad thing for the £300 XPower.
http://www.modders-inc.com/gigabyte-ab350-gaming-3-motherboard-review-fun-flexibility/2/
Probably higher-quality components right there. It is my understanding that the 4+3 layouts are actually weaker than the 4+2 setups. Ironic that MSI would have better VRMs on their product than Gigabyte at this price point.
I'm having problems finding good data on the power delivery for the ASUS Strix B350-F Gaming, though it appears to be a 4+2+2+1 arrangement (effectively 4+2 phase VRM). Probably similar components to what you'll find in the MSI Mortar and Tomahawk.
Anyway, among the three I would definitely put a "do not buy" on that Gigabyte board.
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