So, a "combo" is a fitting where the hubs on the barrel (straight path) are 90 degrees from the side entry, It's a similar pattern to using a wye fitting plus a street 45 for the same spatial arrangement, just molded as a single piece. Anywhere you want to use a wye plus a street 45 you could use a combo instead; if I say wye, but you would need another 45 degrees of bend you can achieve that either way. This is distinct from a sanitary tee, which has less radius on the side entry.
My comment on elevation was that the shower trap arm has to go over the building drain, so the 2" wye where it joins the lav drain is at a higher elevation than the building drain. While the 3" wye where the bathroom group joins the building drain is obviously at the elevation of the building drain. So somewhere between those two, the bathroom branch drain has to drop 3"-5". That elevation change could be upstream of the 3x3x2 (or 3x2x3) wye where the WC joins the shower/lav, or it could be downstream of it. Either is OK.
Either of your drawings is fine, although your renderings of the angles wye/combo fittings don't seem quite right, perhaps as a side effect of trying to do them isometrically. If you are attempting to convey via those angles where the elevation drop occurs in each case, I'm not getting it. I think it would be easier to communicate with a 2-D plan view drawing, where the vertical pipes are just circles, and you just label where the elevation drop occurs.