Through three weeks of the NFL season, a quarter of the teams are still yet to taste defeat.

While some of those were expected to excel - see the New England Patriots, Kansas City Chiefs and Dallas Cowboys - others, such as the Buffalo Bills, San Francisco 49ers and Detroit Lions, have all started strongly after losing seasons in 2018.

This weekend sees the 3-0 Patriots and Bills face off, while the 2-0-1 Lions host Patrick Mahomes and the Chiefs.

Here we take a look at the eight undefeated teams and determine when their first defeat might come, if at all...

DETROIT LIONS (2-0-1)

Upcoming games: vs Chiefs, @ Packers, vs Vikings, vs Giants, @ Raiders, @ Bears

Are the Lions for real? Or did they get fortunate facing a Los Angeles Chargers team that shot themselves in the foot and an injury-ravaged Philadelphia Eagles after their opening tie with the Arizona Cardinals?

Their luck might soon run out, though, as their next three games - against the Chiefs, Packers and Minnesota Vikings - could easily leave them 2-3-1 and behind in the ultra-competitive NFC North.

BUFFALO BILLS (3-0)

Upcoming games: vs Patriots, @ Titans, vs Dolphins, vs Eagles, vs Redskins, @ Browns

Sean McDermott's team nearly suffered their first 'L' on Sunday, only to rally and beat the winless Cincinnati Bengals. 

A daunting clash with the Pats represents a good barometer of where this Bills team is and, even if they go down to Tom Brady and Co, their upcoming contests suggest they can remain firmly in the wildcard mix come November and December.

DALLAS COWBOYS (3-0)

Upcoming games: @ Saints, vs Packers, @ Jets, vs Eagles, @ Giants, vs Vikings

Everything is rosy in JerryWorld at the moment, with coordinator Kellen Moore overseeing a more potent offense thanks to quarterback Dak Prescott taking a significant step forward.

Get past the New Orleans Saints on Sunday Night Football and the Cowboys might march all the way to 10-0 before visiting the Patriots. But New Orleans, coming off an impressive win in Seattle, are a formidable foe at home and should end Dallas' winning streak.

GREEN BAY PACKERS (3-0)

Upcoming games: vs Eagles, @ Cowboys, vs Lions, vs Raiders, @ Chiefs, @ Chargers

Rookie head coach Matt LaFleur is certainly enjoying life at the helm in Titletown and Green Bay host the depleted Eagles this week.

The Pack's offense - LaFleur's speciality - has largely underwhelmed, though, and a Week 5 trip to the talented Cowboys looks likely to spell the end of their unbeaten run.

SAN FRANCISCO 49ERS (3-0)

Upcoming games: vs Browns, @ Rams, @ Redskins, vs Panthers, @ Cardinals, vs Seahawks

San Francisco have the earliest possible bye in Week 4 so their unbeaten run will last until at least Week 5, when they should have too much and be too fresh for a Cleveland Browns team flattering to deceive.

However, after that it's a visit to the Los Angeles Rams, where Sean McVay's offense might prove a bridge too far for the Niners' quietly impressive defense. San Francisco's playoff hopes likely hinge on home games against the Carolina Panthers and Seattle Seahawks.

KANSAS CITY CHIEFS (3-0)

Upcoming games: @ Lions, vs Colts, vs Texans, @ Broncos, vs Packers, vs Vikings

Reigning MVP Mahomes and the Chiefs offense looks even better than it did in 2018 but they have a tricky run of games coming up against teams with playoff aspirations.

A Week 7 Thursday night trip to the Denver Broncos might spell the end of their unbeaten run. Vic Fangio's 0-3 team could conceivably be 2-1 now and Von Miller and Bradley Chubb have to get it together eventually, right?

NEW ENGLAND PATRIOTS (3-0)

Upcoming games: @ Bills, @ Redskins, vs Giants, @ Jets, vs Browns, @ Ravens

New England can thank the schedule-makers for a start that has seen them breeze past the Pittsburgh Steelers, Miami Dolphins and New York Jets - who are a combined 0-9.

A stiffer test against the Bills lies in wait next, but New England have won seven in a row in Buffalo. The Pats could easily be 8-0, but a trip to their nemesis Baltimore - against a Ravens team coming off a bye - might prevent them from winning nine straight.

LOS ANGELES RAMS (3-0)

Upcoming games: vs Buccaneers, @ Seahawks, vs 49ers, @ Falcons, vs Bengals, @ Steelers

Many tipped the Rams to take a blip after their Super Bowl appearance, but here are they are 3-0.

The sternest assignment in their next six games is a trip to Seattle - though New Orleans showed the Seahawks are beatable in the Pacific Northwest. After that, the schedule is kind - particularly now Pittsburgh have lost Ben Roethlisberger - so the Rams look the most likely to be the last unbeaten team standing.