.in 3
First list:

.in 6
.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 6
.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 6
.ti 3
(1)  first item

.ti 3
(2)  second item

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

.in 6
.ti 3
a)  first item

.ti 3
b)  second item

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

.in 6
.ti 3
A.  first item

.ti 3
B.  second item

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

.in 6
.ti 3
1.  first item

.ti 3
2.  second item

.ti 3
3.  third item
.sp 1
A separate line

.in 10
.ti 6
A.  first sub-item

.ti 6
B.  second sub-item

.ti 6
C.  third sub-item