Can a creature with reach block a creature with flying and also a creature without flying in the same combat?

A creature with reach is able to block creatures with flying, just as if it had flying itself (CR 702.17). This means a creature with reach can legally block any attacker that has flying or does not have flying, since reach removes the usual restriction that only flying or reach creatures can block flyers.

However, blocking multiple creatures at once is a separate ability governed by CR 509.1. By default, each blocking creature may only block one attacking creature. To block two attackers simultaneously, the blocking creature would need an ability that specifically allows it to block an additional creature, such as the ability granted by cards like 'can block an additional creature each combat.'

So reach itself does not grant the ability to block multiple creatures — it only grants the ability to block flyers. Without an extra 'block additional creatures' ability, the reach creature must choose exactly one attacker to block, but that attacker can be a flyer or a non-flyer.

Concrete example: You control a Centaur Courser with reach (hypothetically). Your opponent attacks with a 2/2 with flying and a 3/3 with no evasion. Your reach creature can legally choose to block either attacker — the flyer or the ground creature — but cannot block both at the same time unless it has an ability allowing it to block multiple creatures.

In summary: reach expands which creatures can be blocked, but does not expand how many creatures can be blocked simultaneously.