In what way are the 100’s lacking? Potentially, changing to paired 300’s or Fives will be better, or a Sub, depending on the 100s’ inadequacies. 

No complaints. Just asking. Thanks.

Depending on what this is used or would vary what the logical next step would be. if it was for a television the best and only decision would be an arc ultra. depending on price you and location you would need a sub. Therefore, either the sub mini or the GEN four sub not the GEN three (application issues, but if you have the 100 in a separate group it will work with the GEN three). 

Don’t get discouraged when you have to make a separate group from too many units. But remember the ultimate immersive set has two era 300, A GEN four sub and the arc ultra… In my opinion it is completely inadequate without the addition Of 2 era 100s. 
 

— edit try and avoid purchasing a non-arc ultra speaker right now because it seems like there’s going to be a flood of sound motion upgrades on lots of the speakers including the beam, As well as potentially others. (hopefully an early January… I just wish I could pre-order ) But you’re safe on the sub purchasing because the GEN four sub just came out and sound motion is not used for subwoofers

Thank you for your input.

Note: As mentioned here:

Sonos does not recommend grouping “front” speakers with an Arc (or any other soundbar).  The Arc, like all soundbars, already has Left/Center/Right Front speakers.

Quoted from that thread:

Moderator edit: We do not recommend this at all - it will likely sound bad with all channels coming from the “front” speakers.

or the GEN four sub not the GEN three (application issues, but if you have the 100 in a separate group it will work with the GEN three). 
 

​@Zendawn8008 

Is there an issue with the Sub 3 when used with the Arc Ultra?