You are not showing all your code. But you could check if split list is to short, and if so then extend with empty string. Example:

a = '1st St & A blvd'
s = a.split(' & ')
print s

diff = 3-len(s)
if diff != 0:
    s.extend([' ']*diff)
    
print s

Outputs:

['1st St', 'A blvd']
['1st St', 'A blvd', ' ']

Or use try/except:

a = [1,2]
try:
    print a[2]
except IndexError: #a[2] is not possible, will give Indexerror, then dont do anything:
    pass

You are not showing all your code. But you could check if split list is to short, and if so then extend with empty string. Example:

a = '1st St & A blvd'
s = a.split(' & ')
print s

diff = 3-len(s)
if diff != 0:
    s.extend([' ']*diff)
    
print s

Outputs:

['1st St', 'A blvd']
['1st St', 'A blvd', ' ']

Or use try/except:

a = [1,2]
try:
    print a[2]
except IndexError: #a[2] is not possible, will give Indexerror, then dont do anything:
    pass

You could automate everything if you dont mind skipping field calculator:

import arcpy

fc = r'C:\GIS\ArcMap_default_folder\Default.gdb\ak_riks'
splitfield = ['KOMMUNNAMN']

rows = [i[0] for i in arcpy.da.SearchCursor(fc,splitfield)] #List all splitfield values
fields_needed = max([len(r.split('a')) for r in rows]) #Find out how many new fields are needed
prefix = 'Split_' #What to name new fields

newfields = []
for f in range(0,fields_needed):
    newfield = prefix+str(f)
    newfields.append(newfield)
    arcpy.AddField_management(in_table=fc, field_name=newfield, field_type='TEXT', field_length=20) #Adjust field length

with arcpy.da.UpdateCursor(fc, splitfield+newfields) as cursor:
    for row in cursor:
        newdata = row[0].split('a')
        diff = len(newfields) - len(newdata)
        if diff > 0:
            newdata = newdata+[' ']*diff
        else:
            newdata = newdata[:len(newfields)]
        row[1:] = newdata
        cursor.updateRow(row)