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Edit With m = RandomInteger[9, {3, 6, 1}]

Just for completeness:

Catenate /@ m 

Just for diversity reasons

Apply[##&, m, {2}] Apply[Sequence, m, {-2}] 

Using @Mr.Wizard code I see that Catenate /@ m is really good.

Edit With m = RandomInteger[9, {3, 6, 1}]

Just for completeness:

Catenate /@ m 

Just for diversity reasons

Apply[#&, m, {2}] Apply[Sequence, m, {-2}] Map[First, m, {2}] 

Using @Mr.Wizard code I have updated the benchmark.

@MichaelE2 also noticed in comments that Catenate is not compilable.

Edit With m = RandomInteger[9, {3, 6, 1}]

Just for completeness:

Catenate /@ m 

Just for diversity reasons

Apply[##&, m, {2}] Apply[Sequence, m, {-2}] 

Edit With m = RandomInteger[9, {3, 6, 1}]

Just for completeness:

Catenate /@ m 

Just for diversity reasons

Apply[##&, m, {2}] Apply[Sequence, m, {-2}] 

Using @Mr.Wizard code I see that Catenate /@ m is really good.

Just for diversity reasons, with

mat = RandomInteger[9, {3, 6, 1}] 

you may use

Apply[Sequence, mat, {2}] Apply[Sequence, mat, {-2}] 

Edit With m = RandomInteger[9, {3, 6, 1}]

Just for completeness:

Catenate /@ m 

Just for diversity reasons

Apply[##&, m, {2}] Apply[Sequence, m, {-2}]