In SB, only 3bet or fold.

Sounds good.

Yes, makes the preflop play simpler and apparently doesn't lose any value. Even some (a majority?) very good players at med/high stakes do not flat in SB.

I think there are times when it's ok to flat, a hand I just played for example :

Hand History driven straight to this forum with DriveHUD 2

https://drivehud.com/?t=hh

Poker HUD | Stats | Tracking

Poker HUD | Poker Tracking | Online Poker Software | DriveHUD

NL Holdem 0.05(BB)
UTG ($6.57) [VPIP: 10% | PFR: 10% | AGG: 100% | Hands: 10]
HJ ($4.93) [VPIP: 71.4% | PFR: 71.4% | AGG: 66.7% | Flop Agg: 50% | Turn Agg: 100% | River Agg: 100% | 3Bet: 0% | 4Bet: 0% | Hands: 9]
CO ($4.18) [VPIP: 33.3% | PFR: 0% | AGG: 50% | Hands: 6]
BTN ($5.50) [VPIP: 10% | PFR: 10% | AGG: 100% | Hands: 10]
HERO ($5) [VPIP: 24% | PFR: 19.5% | AGG: 35.5% | Flop Agg: 39.4% | Turn Agg: 37.1% | River Agg: 29.8% | 3Bet: 7.3% | 4Bet: 5.8% | Cold Call: 11.4% | Hands: 15801]
BB ($5) [VPIP: 16.9% | PFR: 10.2% | AGG: 46.2% | Hands: 60]

Dealt to Hero: 6♠ 6♥

UTG Folds, HJ Raises To $0.10, CO Folds, BTN Folds, HERO Calls $0.08, BB Folds

Hero SPR on Flop: [19.32 effective]
Flop ($0.25): 7♠ K♦ J♦
HERO Checks, HJ Checks

Turn ($0.25): 7♠ K♦ J♦ 6♦
HERO Bets $0.24 (Rem. Stack: $4.66), HJ Raises To $0.96 (Rem. Stack: $3.87), HERO Calls $0.72 (Rem. Stack: $3.94)

River ($2.17): 7♠ K♦ J♦ 6♦ K♥
HERO Checks, HJ Bets $1.03 (Rem. Stack: $2.84), HERO Raises To $3.94 (allin), HJ Calls $2.84 (allin)

Don't box yourself in to 1 strategy and follow it to the letter. But in general you shouldn't flat from the sb, a smaller raise size and nitty BB makes it better though