#JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for North-West, \$1\$ for North or \$2\$ for North-East.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?' ':c} ${c}
`+g(n,w):''

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#JavaScript (ES6), 90 bytes

This one draws the output character by character with a little more maths instead of the .padStart() method.

Takes input as (direction)(length). Expects \$1\$ for North-West, \$2\$ for North or \$3\$ for North-East.

d=>n=>(g=x=>y?` /|\\
`[x+5?17+y%2*4>>--x+5-[,n+1-y,n,y][d]&1&&d:(x=n,y--,4)]+g(x):'')(y=n)

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JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for North-West, \$1\$ for North or \$2\$ for North-East.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?' ':c} ${c}
`+g(n,w):''

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JavaScript (ES6), 90 bytes

This one draws the output character by character with a little more maths instead of the .padStart() method.

Takes input as (direction)(length). Expects \$1\$ for North-West, \$2\$ for North or \$3\$ for North-East.

d=>n=>(g=x=>y?` /|\\
`[x+5?17+y%2*4>>--x+5-[,n+1-y,n,y][d]&1&&d:(x=n,y--,4)]+g(x):'')(y=n)

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#JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for North-West, \$1\$ for North or \$2\$ for North-East.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?' ':c} ${c}
`+g(n,w):''

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#JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for North-West, \$1\$ for North or \$2\$ for North-East.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?' ':c} ${c}
`+g(n,w):''

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#JavaScript (ES6), 90 bytes

This one draws the output character by character with a little more maths instead of the .padStart() method.

Takes input as (direction)(length). Expects \$1\$ for North-West, \$2\$ for North or \$3\$ for North-East.

d=>n=>(g=x=>y?` /|\\
`[x+5?17+y%2*4>>--x+5-[,n+1-y,n,y][d]&1&&d:(x=n,y--,4)]+g(x):'')(y=n)

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#JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for diagonal, \$1\$ for straight or \$2\$ for anti-diagonal.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?c:' '} ${c}
`+g(n,w):''

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#JavaScript (ES8),  90 87  85 bytes

Takes input as (direction)(length). Expects \$0\$ for North-West, \$1\$ for North or \$2\$ for North-East.

d=>g=(n,w=n)=>n?(c='/|\\'[d]).padStart([n--,,w-n][d])+` ${n&1?' ':c} ${c}
`+g(n,w):''

Try it online!