Well since you have 3 unconditionals you could firm Southampton and even if you missed the offer you woud still get into uni this year. If it was me, I would firm the uni that I really wanted and then have an unconditional as my insurance.
Looking at what you have written it seems that in order of preference you have:
1) Soton
2) Loughborough
3) RHUL
Which subject are you applying for? Because RHUL would be fine if you missed the Soton offer. So personally I would just go with your option 1 (Firm Soton, Insurance RHUL).