.in 3

First list:
.in 7
.ti 3

R1  #1
.ti 3

R2  #2
.ti 3

R3  #3
.in 3

Specify the indent explicitly so that all the items line up nicely.
.sp 1
Different list formatting characters, following what v1 supports:
.sp 2
style="format %i":
.in 10
.ti 3

i.     first item
.ti 3

ii.    second item
.ti 3

iii.   third item
.ti 3

iv.    fourth item
.ti 3

v.     5:th item
.ti 3

vi.    6:th item
.ti 3

vii.   7:th item
.ti 3

viii.  8:th item
.ti 3

ix.    9:th item
.ti 3

x.     10:th item
.ti 3

xi.    11:th item
.ti 3

xii.   12:th item
.ti 3

xiii.  13:th item
.ti 3

xiv.   14:th item
.sp 2
.in 3
style="format (%d)":
.in 8
.ti 3

(1)  first item
.ti 3

(2)  second item
.ti 3

(3)  third item
.sp 2
.in 3
style="format %c)":
.in 7
.ti 3

a)  first item
.ti 3

b)  second item
.ti 3

c)  third item
.sp 2
.in 3
style="format %C.":
.in 7
.ti 3

A.  first item
.ti 3

B.  second item
.ti 3

C.  third item
.sp 2
.in 3
style="format %d.":
.in 7
.ti 3

1.  first item
.ti 3

2.  second item
.ti 3

3.  third item
.sp 1
A separate line
.in 11
.ti 7

A.  first \%sub-item
.ti 7

B.  second \%sub-item
.ti 7

C.  third \%sub-item