Hi Joanne,
I personally wouldn't consider your initial idea of doing all that "before 3 pm" viable because, to me, it's far too rushed. Not only because they're all in different locations -hence the need for allowing some travel time-, but also for each one requires a certain amount of time to visit and not to mention to discover/admire. Yet we arrive to the heart of the matter that so often creates passionate discussions in the forum: quality vs quantity. I lived in Paris for a while and had a good friend working at the Paris Board of Tourism that always use to tell me she had never ceased to be amazed of some of the "flash visits" some visitors made to the Eiffel Tower: "riding up, going around the platform and riding down. Done!" Is really that what traveling from far corners of the world to see Paris and it's famous tower was all about? We all have different travel styles and mine has never been "ticking lists", on the contrary, I always like to plan for unexpected margins which is what I call those little pleasant surprises that unassuming (or not) places give you when you truly discover them: you never thought you'd find "that" there or that I'd be so allured by that "other" thing next to the attraction I was initially going to... so while mathematically yes you could visit all those three things in that time frame -assuming perfect planning, no 'incidents' along the way, absolutely no lunch, and very few toilet stops, lol!- I wouldn't do it myself if I were in your shoes.
Guides vs DIY is another typical discussion all the time. Again, that's very personal and linked to one's own interests and travel style. Yet in cities en vogue like Barcelona -for which you can find numerous excellent guidebooks and other reference materials everywhere (I suppose I should mention RS's if I want to win some brownie points here, LOL!)- and considering the sheer number of attractions, sights and things worthwhile visiting at your tips across the whole city, I would be inclined to recommend DIY visits. Besides, in Barcelona many of the most popular attractions have audio guides available (in a range of languages) and/or informative panels (English is always included as third language in most of them) so hiring a guide person is not essential. I do appreciate of course that a good guide person might bring much more than mere basic guidance.
As per moving about, as said English is nearly always present as third language (transportation, restaurants...) so one shouldn't have to have a problem finding one's way. Furthermore, and taking into account the number of visitors from all over the world we receive every year, for the past 30 years our City Hall has gradually been creating a very iconographic communication policy -much like other cities like NY or Paris- so many of those visitors that don't speak neither Catalan nor Spanish and might not have a good command of English either, can equally navigate the city, take the metro or find out where things are located in an easy manner.
My advise for Barcelona is simple: define your goals and interests, do a bit of research on the city and the culture, focus on those things that might suit those interests, make a list of must-see, would-like-to-see and sounds-interesting so you can prioritize according to each circumstance, check maps for distances and websites for schedules (and costs!), draft an itinerary and head to a travel forum for validation and advise.
So... before giving you specific practical advise on your questions, may I ask your rough itinerary for the days you're going to be in Barcelona?, knowing what you want to see it's much easier to shuffle stuff when necessary and see what can be best fitted where (time-wise) so you can better optimize your time. It'd be also useful to know if you have already time constrictions (maybe you have already a ticket for a given time, or you have to be somewhere at a specific date/time...)